diff --git a/readme.md b/readme.md
index 2b26cbdd1a..4fe76a2faf 100644
--- a/readme.md
+++ b/readme.md
@@ -18,7 +18,7 @@ Avalonia is a WPF-inspired cross-platform XAML-based UI framework providing a fl
 
 ## Getting Started
 
-Avalonia [Visual Studio Extension](https://marketplace.visualstudio.com/items?itemName=AvaloniaTeam.AvaloniaforVisualStudio) contains project and control templates that will help you get started. After installing it, open "New Project" dialog in Visual Studio, choose "Avalonia" in "Visual C#" section, select "Avalonia .NET Core Application" and press OK (<a href="http://avaloniaui.net/tutorial/images/add-dialogs.png">screenshot</a>). Now you can write code and markup that will work on multiple platforms!
+Avalonia [Visual Studio Extension](https://marketplace.visualstudio.com/items?itemName=AvaloniaTeam.AvaloniaforVisualStudio) contains project and control templates that will help you get started. After installing it, open "New Project" dialog in Visual Studio, choose "Avalonia" in "Visual C#" section, select "Avalonia .NET Core Application" and press OK (<a href="http://avaloniaui.net/docs/quickstart/images/new-project-dialog.png">screenshot</a>). Now you can write code and markup that will work on multiple platforms!
 
 Avalonia is delivered via <b>NuGet</b> package manager. You can find the packages here: ([stable(ish)](https://www.nuget.org/packages/Avalonia/), [nightly](https://github.com/AvaloniaUI/Avalonia/wiki/Using-nightly-build-feed))
 
@@ -52,7 +52,7 @@ Please read the [contribution guidelines](http://avaloniaui.net/contributing/con
 ### Contributors
 
 This project exists thanks to all the people who contribute. [[Contribute](http://avaloniaui.net/contributing/contributing)].
-<a href="graphs/contributors"><img src="https://opencollective.com/Avalonia/contributors.svg?width=890&button=false" /></a>
+<a href="https://github.com/AvaloniaUI/Avalonia/graphs/contributors"><img src="https://opencollective.com/Avalonia/contributors.svg?width=890&button=false" /></a>
 
 
 ### Backers
diff --git a/samples/ControlCatalog/DecoratedWindow.xaml.cs b/samples/ControlCatalog/DecoratedWindow.xaml.cs
index d28281e476..749f83c1ab 100644
--- a/samples/ControlCatalog/DecoratedWindow.xaml.cs
+++ b/samples/ControlCatalog/DecoratedWindow.xaml.cs
@@ -20,7 +20,7 @@ namespace ControlCatalog
             ctl.Cursor = new Cursor(cursor);
             ctl.PointerPressed += delegate
             {
-                PlatformImpl.BeginResizeDrag(edge);
+                PlatformImpl?.BeginResizeDrag(edge);
             };
         }
 
@@ -29,7 +29,7 @@ namespace ControlCatalog
             AvaloniaXamlLoader.Load(this);
             this.FindControl<Control>("TitleBar").PointerPressed += delegate
             {
-                PlatformImpl.BeginMoveDrag();
+                PlatformImpl?.BeginMoveDrag();
             };
             SetupSide("Left", StandardCursorType.LeftSide, WindowEdge.West);
             SetupSide("Right", StandardCursorType.RightSide, WindowEdge.East);
diff --git a/src/Android/Avalonia.Android/Platform/SkiaPlatform/PopupImpl.cs b/src/Android/Avalonia.Android/Platform/SkiaPlatform/PopupImpl.cs
index 78f744cea0..041d91043a 100644
--- a/src/Android/Avalonia.Android/Platform/SkiaPlatform/PopupImpl.cs
+++ b/src/Android/Avalonia.Android/Platform/SkiaPlatform/PopupImpl.cs
@@ -110,7 +110,10 @@ namespace Avalonia.Android.Platform.SkiaPlatform
         {
             //Not supported
         }
-        
 
+        public void SetTopmost(bool value)
+        {
+            //Not supported
+        }
     }
 }
\ No newline at end of file
diff --git a/src/Avalonia.Base/Data/Core/ExpressionNode.cs b/src/Avalonia.Base/Data/Core/ExpressionNode.cs
index ae70cacdba..ac7e97a4b1 100644
--- a/src/Avalonia.Base/Data/Core/ExpressionNode.cs
+++ b/src/Avalonia.Base/Data/Core/ExpressionNode.cs
@@ -11,6 +11,7 @@ namespace Avalonia.Data.Core
 {
     internal abstract class ExpressionNode : ISubject<object>
     {
+        private static readonly object CacheInvalid = new object();
         protected static readonly WeakReference UnsetReference = 
             new WeakReference(AvaloniaProperty.UnsetValue);
 
@@ -18,6 +19,8 @@ namespace Avalonia.Data.Core
         private IDisposable _valueSubscription;
         private IObserver<object> _observer;
 
+        protected WeakReference LastValue { get; private set; }
+
         public abstract string Description { get; }
         public ExpressionNode Next { get; set; }
 
@@ -61,6 +64,7 @@ namespace Avalonia.Data.Core
             {
                 _valueSubscription?.Dispose();
                 _valueSubscription = null;
+                LastValue = null;
                 nextSubscription?.Dispose();
                 _observer = null;
             });
@@ -120,6 +124,7 @@ namespace Avalonia.Data.Core
 
             if (notification == null)
             {
+                LastValue = new WeakReference(value);
                 if (Next != null)
                 {
                     Next.Target = new WeakReference(value);
@@ -131,6 +136,7 @@ namespace Avalonia.Data.Core
             }
             else
             {
+                LastValue = new WeakReference(notification.Value);
                 if (Next != null)
                 {
                     Next.Target = new WeakReference(notification.Value);
diff --git a/src/Avalonia.Base/Data/Core/ExpressionObserver.cs b/src/Avalonia.Base/Data/Core/ExpressionObserver.cs
index 7719f93a02..14bc09f5b7 100644
--- a/src/Avalonia.Base/Data/Core/ExpressionObserver.cs
+++ b/src/Avalonia.Base/Data/Core/ExpressionObserver.cs
@@ -154,7 +154,7 @@ namespace Avalonia.Data.Core
         /// </returns>
         public bool SetValue(object value, BindingPriority priority = BindingPriority.LocalValue)
         {
-            if (Leaf is ISettableNode settable)
+            if (Leaf is SettableNode settable)
             {
                 var node = _node;
                 while (node != null)
@@ -188,7 +188,7 @@ namespace Avalonia.Data.Core
         /// Gets the type of the expression result or null if the expression could not be 
         /// evaluated.
         /// </summary>
-        public Type ResultType => (Leaf as ISettableNode)?.PropertyType;
+        public Type ResultType => (Leaf as SettableNode)?.PropertyType;
 
         /// <summary>
         /// Gets the leaf node.
diff --git a/src/Avalonia.Base/Data/Core/ISettableNode.cs b/src/Avalonia.Base/Data/Core/ISettableNode.cs
deleted file mode 100644
index 7788407833..0000000000
--- a/src/Avalonia.Base/Data/Core/ISettableNode.cs
+++ /dev/null
@@ -1,15 +0,0 @@
-using Avalonia.Data;
-using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
-using System.Threading.Tasks;
-
-namespace Avalonia.Data.Core
-{
-    interface ISettableNode
-    {
-        bool SetTargetValue(object value, BindingPriority priority);
-        Type PropertyType { get; }
-    }
-}
diff --git a/src/Avalonia.Base/Data/Core/IndexerNode.cs b/src/Avalonia.Base/Data/Core/IndexerNode.cs
index 47e82fa2d3..633d3558ee 100644
--- a/src/Avalonia.Base/Data/Core/IndexerNode.cs
+++ b/src/Avalonia.Base/Data/Core/IndexerNode.cs
@@ -15,7 +15,7 @@ using Avalonia.Data;
 
 namespace Avalonia.Data.Core
 {
-    internal class IndexerNode : ExpressionNode, ISettableNode
+    internal class IndexerNode :  SettableNode
     {
         public IndexerNode(IList<string> arguments)
         {
@@ -52,7 +52,7 @@ namespace Avalonia.Data.Core
             return Observable.Merge(inputs).StartWith(GetValue(target));
         }
 
-        public bool SetTargetValue(object value, BindingPriority priority)
+        protected override bool SetTargetValueCore(object value, BindingPriority priority)
         {
             var typeInfo = Target.Target.GetType().GetTypeInfo();
             var list = Target.Target as IList;
@@ -154,7 +154,7 @@ namespace Avalonia.Data.Core
 
         public IList<string> Arguments { get; }
 
-        public Type PropertyType => GetIndexer(Target.Target.GetType().GetTypeInfo())?.PropertyType;
+        public override Type PropertyType => GetIndexer(Target.Target.GetType().GetTypeInfo())?.PropertyType;
 
         private object GetValue(object target)
         {
diff --git a/src/Avalonia.Base/Data/Core/PropertyAccessorNode.cs b/src/Avalonia.Base/Data/Core/PropertyAccessorNode.cs
index 4dbff4602f..9d657b3144 100644
--- a/src/Avalonia.Base/Data/Core/PropertyAccessorNode.cs
+++ b/src/Avalonia.Base/Data/Core/PropertyAccessorNode.cs
@@ -10,7 +10,7 @@ using Avalonia.Data.Core.Plugins;
 
 namespace Avalonia.Data.Core
 {
-    internal class PropertyAccessorNode : ExpressionNode, ISettableNode
+    internal class PropertyAccessorNode : SettableNode
     {
         private readonly bool _enableValidation;
         private IPropertyAccessor _accessor;
@@ -23,13 +23,17 @@ namespace Avalonia.Data.Core
 
         public override string Description => PropertyName;
         public string PropertyName { get; }
-        public Type PropertyType => _accessor?.PropertyType;
+        public override Type PropertyType => _accessor?.PropertyType;
 
-        public bool SetTargetValue(object value, BindingPriority priority)
+        protected override bool SetTargetValueCore(object value, BindingPriority priority)
         {
             if (_accessor != null)
             {
-                try { return _accessor.SetValue(value, priority); } catch { }
+                try
+                {
+                    return _accessor.SetValue(value, priority);
+                }
+                catch { }
             }
 
             return false;
@@ -56,7 +60,10 @@ namespace Avalonia.Data.Core
                 () =>
                 {
                     _accessor = accessor;
-                    return Disposable.Create(() => _accessor = null);
+                    return Disposable.Create(() =>
+                    {
+                        _accessor = null;
+                    });
                 },
                 _ => accessor);
         }
diff --git a/src/Avalonia.Base/Data/Core/SettableNode.cs b/src/Avalonia.Base/Data/Core/SettableNode.cs
new file mode 100644
index 0000000000..092cdbe48f
--- /dev/null
+++ b/src/Avalonia.Base/Data/Core/SettableNode.cs
@@ -0,0 +1,38 @@
+using Avalonia.Data;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Avalonia.Data.Core
+{
+    internal abstract class SettableNode : ExpressionNode
+    {
+        public bool SetTargetValue(object value, BindingPriority priority)
+        {
+            if (ShouldNotSet(value))
+            {
+                return true;
+            }
+            return SetTargetValueCore(value, priority);
+        }
+
+        private bool ShouldNotSet(object value)
+        {
+            if (PropertyType == null)
+            {
+                return false;
+            }
+            if (PropertyType.IsValueType)
+            {
+                return LastValue?.Target != null && LastValue.Target.Equals(value);
+            }
+            return LastValue != null && Object.ReferenceEquals(LastValue?.Target, value);
+        }
+
+        protected abstract bool SetTargetValueCore(object value, BindingPriority priority);
+
+        public abstract Type PropertyType { get; }
+    }
+}
diff --git a/src/Avalonia.Controls/AutoCompleteBox.cs b/src/Avalonia.Controls/AutoCompleteBox.cs
index 351cbf5520..96fb9be8ac 100644
--- a/src/Avalonia.Controls/AutoCompleteBox.cs
+++ b/src/Avalonia.Controls/AutoCompleteBox.cs
@@ -2150,7 +2150,7 @@ namespace Avalonia.Controls
             }
 
             // Update the view
-            if (e.Action == NotifyCollectionChangedAction.Remove || e.Action == NotifyCollectionChangedAction.Replace)
+            if ((e.Action == NotifyCollectionChangedAction.Remove || e.Action == NotifyCollectionChangedAction.Replace) && e.OldItems != null)
             {
                 for (int index = 0; index < e.OldItems.Count; index++)
                 {
diff --git a/src/Avalonia.Controls/Calendar/Calendar.cs b/src/Avalonia.Controls/Calendar/Calendar.cs
index 59281c5ad0..029e4dadc8 100644
--- a/src/Avalonia.Controls/Calendar/Calendar.cs
+++ b/src/Avalonia.Controls/Calendar/Calendar.cs
@@ -769,7 +769,7 @@ namespace Avalonia.Controls
         }
         private static void UpdateDisplayDate(Calendar c, DateTime addedDate, DateTime removedDate)
         {
-            Debug.Assert(c != null, "c should not be null!");
+            Contract.Requires<ArgumentNullException>(c != null);
 
             // If DisplayDate < DisplayDateStart, DisplayDate = DisplayDateStart
             if (DateTime.Compare(addedDate, c.DisplayDateRangeStart) < 0)
@@ -1016,8 +1016,6 @@ namespace Avalonia.Controls
 
         internal CalendarDayButton FindDayButtonFromDay(DateTime day)
         {
-            CalendarDayButton b;
-            DateTime? d;
             CalendarItem monthControl = MonthControl;
 
             // REMOVE_RTM: should be updated if we support MultiCalendar
@@ -1028,14 +1026,16 @@ namespace Avalonia.Controls
                 {
                     for (int childIndex = ColumnsPerMonth; childIndex < count; childIndex++)
                     {
-                        b = monthControl.MonthView.Children[childIndex] as CalendarDayButton;
-                        d = b.DataContext as DateTime?;
-
-                        if (d.HasValue)
+                        if (monthControl.MonthView.Children[childIndex] is CalendarDayButton b)
                         {
-                            if (DateTimeHelper.CompareDays(d.Value, day) == 0)
+                            var d = b.DataContext as DateTime?;
+
+                            if (d.HasValue)
                             {
-                                return b;
+                                if (DateTimeHelper.CompareDays(d.Value, day) == 0)
+                                {
+                                    return b;
+                                }
                             }
                         }
                     }
@@ -1044,20 +1044,6 @@ namespace Avalonia.Controls
             return null;
         }
 
-        private void Calendar_SizeChanged(object sender, EventArgs e)
-        {
-            Debug.Assert(sender is Calendar, "The sender should be a Calendar!");
-
-            var size = Bounds.Size;
-            RectangleGeometry rg = new RectangleGeometry();
-            rg.Rect = new Rect(0, 0, size.Width, size.Height);
-
-            if (Root != null)
-            {
-                Root.Clip = rg;
-            }
-        }
-
         private void OnSelectedMonthChanged(DateTime? selectedMonth)
         {
             if (selectedMonth.HasValue)
@@ -1090,7 +1076,6 @@ namespace Avalonia.Controls
 
         internal void ResetStates()
         {
-            CalendarDayButton d;
             CalendarItem monthControl = MonthControl;
             int count = RowsPerMonth * ColumnsPerMonth;
             if (monthControl != null)
@@ -1099,7 +1084,7 @@ namespace Avalonia.Controls
                 {
                     for (int childIndex = ColumnsPerMonth; childIndex < count; childIndex++)
                     {
-                        d = monthControl.MonthView.Children[childIndex] as CalendarDayButton;
+                        var d = (CalendarDayButton)monthControl.MonthView.Children[childIndex];
                         d.IgnoreMouseOverState();
                     }
                 }
@@ -1190,8 +1175,6 @@ namespace Avalonia.Controls
             if (HoverEnd != null && HoverStart != null)
             {
                 int startIndex, endIndex, i;
-                CalendarDayButton b;
-                DateTime? d;
                 CalendarItem monthControl = MonthControl;
 
                 // This assumes a contiguous set of dates:
@@ -1201,18 +1184,20 @@ namespace Avalonia.Controls
 
                     for (i = startIndex; i <= endIndex; i++)
                     {
-                        b = monthControl.MonthView.Children[i] as CalendarDayButton;
-                        b.IsSelected = true;
-                        d = b.DataContext as DateTime?;
-
-                        if (d.HasValue && DateTimeHelper.CompareDays(HoverEnd.Value, d.Value) == 0)
+                        if (monthControl.MonthView.Children[i] is CalendarDayButton b)
                         {
-                            if (FocusButton != null)
+                            b.IsSelected = true;
+                            var d = b.DataContext as DateTime?;
+
+                            if (d.HasValue && DateTimeHelper.CompareDays(HoverEnd.Value, d.Value) == 0)
                             {
-                                FocusButton.IsCurrent = false;
+                                if (FocusButton != null)
+                                {
+                                    FocusButton.IsCurrent = false;
+                                }
+                                b.IsCurrent = HasFocusInternal;
+                                FocusButton = b;
                             }
-                            b.IsCurrent = HasFocusInternal;
-                            FocusButton = b;
                         }
                     }
                 }
@@ -1228,8 +1213,6 @@ namespace Avalonia.Controls
             if (HoverEnd != null && HoverStart != null)
             {
                 CalendarItem monthControl = MonthControl;
-                CalendarDayButton b;
-                DateTime? d;
 
                 if (HoverEndIndex != null && HoverStartIndex != null)
                 {
@@ -1240,15 +1223,17 @@ namespace Avalonia.Controls
                     {
                         for (i = startIndex; i <= endIndex; i++)
                         {
-                            b = monthControl.MonthView.Children[i] as CalendarDayButton;
-                            d = b.DataContext as DateTime?;
-
-                            if (d.HasValue)
+                            if (monthControl.MonthView.Children[i] is CalendarDayButton b)
                             {
-                                if (!SelectedDates.Contains(d.Value))
+                                var d = b.DataContext as DateTime?;
+
+                                if (d.HasValue)
                                 {
-                                    b.IsSelected = false;
-                                }
+                                    if (!SelectedDates.Contains(d.Value))
+                                    {
+                                        b.IsSelected = false;
+                                    }
+                                } 
                             }
                         }
                     }
@@ -1257,7 +1242,7 @@ namespace Avalonia.Controls
                         // It is SingleRange
                         for (i = startIndex; i <= endIndex; i++)
                         {
-                            (monthControl.MonthView.Children[i] as CalendarDayButton).IsSelected = false;
+                            ((CalendarDayButton)monthControl.MonthView.Children[i]).IsSelected = false;
                         }
                     }
                 }
@@ -1628,16 +1613,15 @@ namespace Avalonia.Controls
                 e.Handled = true;
             }
         }
-        internal void Calendar_KeyDown(object sender, KeyEventArgs e)
-        {
-            Calendar c = sender as Calendar;
-            Debug.Assert(c != null, "c should not be null!");
 
-            if (!e.Handled && c.IsEnabled)
+        internal void Calendar_KeyDown(KeyEventArgs e)
+        {
+            if (!e.Handled && IsEnabled)
             {
                 e.Handled = ProcessCalendarKey(e);
             }
         }
+
         internal bool ProcessCalendarKey(KeyEventArgs e)
         {
             if (DisplayMode == CalendarMode.Month)
@@ -1976,7 +1960,7 @@ namespace Avalonia.Controls
                     }
             }
         }
-        private void Calendar_KeyUp(object sender, KeyEventArgs e)
+        private void Calendar_KeyUp(KeyEventArgs e)
         {
             if (!e.Handled && (e.Key == Key.LeftShift || e.Key == Key.RightShift))
             {
@@ -2083,6 +2067,9 @@ namespace Avalonia.Controls
             DisplayDateProperty.Changed.AddClassHandler<Calendar>(x => x.OnDisplayDateChanged);
             DisplayDateStartProperty.Changed.AddClassHandler<Calendar>(x => x.OnDisplayDateStartChanged);
             DisplayDateEndProperty.Changed.AddClassHandler<Calendar>(x => x.OnDisplayDateEndChanged);
+            KeyDownEvent.AddClassHandler<Calendar>(x => x.Calendar_KeyDown);
+            KeyUpEvent.AddClassHandler<Calendar>(x => x.Calendar_KeyUp);
+            
         }
 
         /// <summary>
@@ -2122,10 +2109,6 @@ namespace Avalonia.Controls
                     month.Owner = this;
                 }
             }
-
-            LayoutUpdated += Calendar_SizeChanged;
-            KeyDown += Calendar_KeyDown;
-            KeyUp += Calendar_KeyUp;
         }
 
     }
diff --git a/src/Avalonia.Controls/Calendar/CalendarDateRange.cs b/src/Avalonia.Controls/Calendar/CalendarDateRange.cs
index 273cda8c5b..718cc7142b 100644
--- a/src/Avalonia.Controls/Calendar/CalendarDateRange.cs
+++ b/src/Avalonia.Controls/Calendar/CalendarDateRange.cs
@@ -66,7 +66,7 @@ namespace Avalonia.Controls
         /// <returns>Inherited code: Requires comment 2.</returns>
         internal bool ContainsAny(CalendarDateRange range)
         {
-            Debug.Assert(range != null, "range should not be null!");
+            Contract.Requires<ArgumentNullException>(range != null);
 
             int start = DateTime.Compare(Start, range.Start);
 
diff --git a/src/Avalonia.Controls/Calendar/CalendarItem.cs b/src/Avalonia.Controls/Calendar/CalendarItem.cs
index 3432fa549d..b0cbd0be53 100644
--- a/src/Avalonia.Controls/Calendar/CalendarItem.cs
+++ b/src/Avalonia.Controls/Calendar/CalendarItem.cs
@@ -517,7 +517,7 @@ namespace Avalonia.Controls.Primitives
             for (int childIndex = Calendar.ColumnsPerMonth; childIndex < count; childIndex++)
             {
                 CalendarDayButton childButton = MonthView.Children[childIndex] as CalendarDayButton;
-                Debug.Assert(childButton != null, "childButton should not be null!");
+                Contract.Requires<ArgumentNullException>(childButton != null);
 
                 childButton.Index = childIndex;
                 SetButtonState(childButton, dateToAdd);
@@ -554,7 +554,7 @@ namespace Avalonia.Controls.Primitives
                     for (int i = childIndex; i < count; i++)
                     {
                         childButton = MonthView.Children[i] as CalendarDayButton;
-                        Debug.Assert(childButton != null, "childButton should not be null!");
+                        Contract.Requires<ArgumentNullException>(childButton != null);
                         // button needs a content to occupy the necessary space
                         // for the content presenter
                         childButton.Content = i.ToString(DateTimeHelper.GetCurrentDateFormat());
@@ -650,7 +650,7 @@ namespace Avalonia.Controls.Primitives
             foreach (object child in YearView.Children)
             {
                 CalendarButton childButton = child as CalendarButton;
-                Debug.Assert(childButton != null, "childButton should not be null!");
+                Contract.Requires<ArgumentNullException>(childButton != null);
                 // There should be no time component. Time is 12:00 AM
                 DateTime day = new DateTime(_currentMonth.Year, count + 1, 1);
                 childButton.DataContext = day;
@@ -746,7 +746,7 @@ namespace Avalonia.Controls.Primitives
             foreach (object child in YearView.Children)
             {
                 CalendarButton childButton = child as CalendarButton;
-                Debug.Assert(childButton != null, "childButton should not be null!");
+                Contract.Requires<ArgumentNullException>(childButton != null);
                 year = decade + count;
 
                 if (year <= DateTime.MaxValue.Year && year >= DateTime.MinValue.Year)
@@ -826,7 +826,7 @@ namespace Avalonia.Controls.Primitives
                 {
                     Owner.Focus();
                 }
-                Button b = sender as Button;
+                Button b = (Button)sender;
                 DateTime d;
 
                 if (b.IsEnabled)
@@ -863,7 +863,7 @@ namespace Avalonia.Controls.Primitives
                     Owner.Focus();
                 }
 
-                Button b = sender as Button;
+                Button b = (Button)sender;
                 if (b.IsEnabled)
                 {
                     Owner.OnPreviousClick();
@@ -878,7 +878,7 @@ namespace Avalonia.Controls.Primitives
                 {
                     Owner.Focus();
                 }
-                Button b = sender as Button;
+                Button b = (Button)sender;
 
                 if (b.IsEnabled)
                 {
@@ -891,8 +891,7 @@ namespace Avalonia.Controls.Primitives
         {
             if (Owner != null)
             {
-                CalendarDayButton b = sender as CalendarDayButton;
-                if (_isMouseLeftButtonDown && b != null && b.IsEnabled && !b.IsBlackout)
+                if (_isMouseLeftButtonDown && sender is CalendarDayButton b && b.IsEnabled && !b.IsBlackout)
                 {
                     // Update the states of all buttons to be selected starting
                     // from HoverStart to b
@@ -918,7 +917,7 @@ namespace Avalonia.Controls.Primitives
                                 Debug.Assert(b.DataContext != null, "The DataContext should not be null!");
                                 Owner.UnHighlightDays();
                                 Owner.HoverEndIndex = b.Index;
-                                Owner.HoverEnd = (DateTime)b.DataContext;
+                                Owner.HoverEnd = (DateTime?)b.DataContext;
                                 // Update the States of the buttons
                                 Owner.HighlightDays();
                                 return;
@@ -931,7 +930,7 @@ namespace Avalonia.Controls.Primitives
         {
             if (_isMouseLeftButtonDown)
             {
-                CalendarDayButton b = sender as CalendarDayButton;
+                CalendarDayButton b = (CalendarDayButton)sender;
                 // The button is in Pressed state. Change the state to normal.
                 if (e.Device.Captured == b)
                     e.Device.Capture(null);
@@ -973,7 +972,7 @@ namespace Avalonia.Controls.Primitives
                     if (b.IsEnabled && !b.IsBlackout)
                     {
                         DateTime selectedDate = (DateTime)b.DataContext;
-                        Debug.Assert(selectedDate != null, "selectedDate should not be null!");
+                        Contract.Requires<ArgumentNullException>(selectedDate != null);
                         _isMouseLeftButtonDown = true;
                         // null check is added for unit tests
                         if (e != null)
@@ -1149,7 +1148,7 @@ namespace Avalonia.Controls.Primitives
                 if (_isControlPressed && Owner.SelectionMode == CalendarSelectionMode.MultipleRange)
                 {
                     CalendarDayButton b = sender as CalendarDayButton;
-                    Debug.Assert(b != null, "The sender should be a non-null CalendarDayButton!");
+                    Contract.Requires<ArgumentNullException>(b != null);
 
                     if (b.IsSelected)
                     {
@@ -1169,7 +1168,7 @@ namespace Avalonia.Controls.Primitives
         private void Month_CalendarButtonMouseDown(object sender, PointerPressedEventArgs e)
         {
             CalendarButton b = sender as CalendarButton;
-            Debug.Assert(b != null, "The sender should be a non-null CalendarDayButton!");
+            Contract.Requires<ArgumentNullException>(b != null);
 
             _isMouseLeftButtonDownYearView = true;
 
@@ -1208,7 +1207,7 @@ namespace Avalonia.Controls.Primitives
             if (_isMouseLeftButtonDownYearView)
             {
                 CalendarButton b = sender as CalendarButton;
-                Debug.Assert(b != null, "The sender should be a non-null CalendarDayButton!");
+                Contract.Requires<ArgumentNullException>(b != null);
                 UpdateYearViewSelection(b);
             }
         }
@@ -1217,7 +1216,7 @@ namespace Avalonia.Controls.Primitives
         {
             if (_isMouseLeftButtonDownYearView)
             {
-                CalendarButton b = sender as CalendarButton;
+                CalendarButton b = (CalendarButton)sender;
                 // The button is in Pressed state. Change the state to normal.
                 if (e.Device.Captured == b)
                     e.Device.Capture(null);
diff --git a/src/Avalonia.Controls/Calendar/DatePicker.cs b/src/Avalonia.Controls/Calendar/DatePicker.cs
index 418ef50b2c..08608ad359 100644
--- a/src/Avalonia.Controls/Calendar/DatePicker.cs
+++ b/src/Avalonia.Controls/Calendar/DatePicker.cs
@@ -842,7 +842,7 @@ namespace Avalonia.Controls
         private void Calendar_KeyDown(object sender, KeyEventArgs e)
         {
             Calendar c = sender as Calendar;
-            Debug.Assert(c != null, "The Calendar should not be null!");
+            Contract.Requires<ArgumentNullException>(c != null);
 
             if (!e.Handled && (e.Key == Key.Enter || e.Key == Key.Space || e.Key == Key.Escape) && c.DisplayMode == CalendarMode.Month)
             {
diff --git a/src/Avalonia.Controls/DropDownItem.cs b/src/Avalonia.Controls/DropDownItem.cs
index 3fd80c4562..fb465e93ec 100644
--- a/src/Avalonia.Controls/DropDownItem.cs
+++ b/src/Avalonia.Controls/DropDownItem.cs
@@ -22,14 +22,13 @@ namespace Avalonia.Controls
         static DropDownItem()
         {
             FocusableProperty.OverrideDefaultValue<DropDownItem>(true);
-            IsFocusedProperty.Changed.Subscribe(x =>
-            {
-                var sender = x.Sender as IControl;
+        }
 
-                if (sender != null)
-                {
-                    ((IPseudoClasses)sender.Classes).Set(":selected", (bool)x.NewValue);
-                }
+        public DropDownItem()
+        {
+            this.GetObservable(DropDownItem.IsFocusedProperty).Subscribe(focused =>
+            {
+                PseudoClasses.Set(":selected", focused);                
             });
         }
 
diff --git a/src/Avalonia.Controls/Embedding/Offscreen/OffscreenTopLevel.cs b/src/Avalonia.Controls/Embedding/Offscreen/OffscreenTopLevel.cs
index cd2da692ba..4db16c71a5 100644
--- a/src/Avalonia.Controls/Embedding/Offscreen/OffscreenTopLevel.cs
+++ b/src/Avalonia.Controls/Embedding/Offscreen/OffscreenTopLevel.cs
@@ -57,7 +57,7 @@ namespace Avalonia.Controls.Embedding.Offscreen
         Type IStyleable.StyleKey => typeof(EmbeddableControlRoot);
         public void Dispose()
         {
-            PlatformImpl.Dispose();
+            PlatformImpl?.Dispose();
         }
     }
 }
diff --git a/src/Avalonia.Controls/Platform/IWindowBaseImpl.cs b/src/Avalonia.Controls/Platform/IWindowBaseImpl.cs
index 4f7ac82df7..9ba68f584e 100644
--- a/src/Avalonia.Controls/Platform/IWindowBaseImpl.cs
+++ b/src/Avalonia.Controls/Platform/IWindowBaseImpl.cs
@@ -72,6 +72,11 @@ namespace Avalonia.Platform
         /// 
         void SetMinMaxSize(Size minSize, Size maxSize);
 
+        /// <summary>
+        /// Sets whether this window appears on top of all other windows
+        /// </summary>
+        void SetTopmost(bool value);
+
         /// <summary>
         /// Gets platform specific display information
         /// </summary>
diff --git a/src/Avalonia.Controls/Platform/InProcessDragSource.cs b/src/Avalonia.Controls/Platform/InProcessDragSource.cs
index e136efe2a9..52e09aba45 100644
--- a/src/Avalonia.Controls/Platform/InProcessDragSource.cs
+++ b/src/Avalonia.Controls/Platform/InProcessDragSource.cs
@@ -62,7 +62,7 @@ namespace Avalonia.Platform
 
             RawDragEvent rawEvent = new RawDragEvent(_dragDrop, type, root, pt, _draggedData, _allowedEffects);
             var tl = root.GetSelfAndVisualAncestors().OfType<TopLevel>().FirstOrDefault();
-            tl.PlatformImpl.Input(rawEvent);
+            tl.PlatformImpl?.Input(rawEvent);
 
             var effect = GetPreferredEffect(rawEvent.Effects & _allowedEffects, modifiers);
             UpdateCursor(root, effect);
diff --git a/src/Avalonia.Controls/Presenters/CarouselPresenter.cs b/src/Avalonia.Controls/Presenters/CarouselPresenter.cs
index 144174e371..e438843078 100644
--- a/src/Avalonia.Controls/Presenters/CarouselPresenter.cs
+++ b/src/Avalonia.Controls/Presenters/CarouselPresenter.cs
@@ -126,7 +126,21 @@ namespace Avalonia.Controls.Presenters
                         generator.Clear();
                         Panel.Children.RemoveAll(containers.Select(x => x.ContainerControl));
 
-                        MoveToPage(-1, SelectedIndex >= 0 ? SelectedIndex : 0);
+                        var newIndex = SelectedIndex;
+
+                        if(SelectedIndex < 0)
+                        {
+                            if(Items != null && Items.Count() > 0)
+                            {
+                                newIndex = 0;
+                            }
+                            else
+                            {
+                                newIndex = -1;
+                            }
+                        }
+                        
+                        MoveToPage(-1, newIndex);
                     }
                     break;     
             }
diff --git a/src/Avalonia.Controls/Primitives/Popup.cs b/src/Avalonia.Controls/Primitives/Popup.cs
index 656f3890cd..005717d681 100644
--- a/src/Avalonia.Controls/Primitives/Popup.cs
+++ b/src/Avalonia.Controls/Primitives/Popup.cs
@@ -70,6 +70,12 @@ namespace Avalonia.Controls.Primitives
         public static readonly StyledProperty<bool> StaysOpenProperty =
             AvaloniaProperty.Register<Popup, bool>(nameof(StaysOpen), true);
 
+        /// <summary>
+        /// Defines the <see cref="Topmost"/> property.
+        /// </summary>
+        public static readonly StyledProperty<bool> TopmostProperty =
+            AvaloniaProperty.Register<Popup, bool>(nameof(Topmost));
+
         private bool _isOpen;
         private PopupRoot _popupRoot;
         private TopLevel _topLevel;
@@ -84,6 +90,7 @@ namespace Avalonia.Controls.Primitives
             IsHitTestVisibleProperty.OverrideDefaultValue<Popup>(false);
             ChildProperty.Changed.AddClassHandler<Popup>(x => x.ChildChanged);
             IsOpenProperty.Changed.AddClassHandler<Popup>(x => x.IsOpenChanged);
+            TopmostProperty.Changed.AddClassHandler<Popup>((p, e) => p.PopupRoot.Topmost = (bool)e.NewValue);
         }
 
         /// <summary>
@@ -194,6 +201,15 @@ namespace Avalonia.Controls.Primitives
             set { SetValue(StaysOpenProperty, value); }
         }
 
+        /// <summary>
+        /// Gets or sets whether this popup appears on top of all other windows
+        /// </summary>
+        public bool Topmost
+        {
+            get { return GetValue(TopmostProperty); }
+            set { SetValue(TopmostProperty, value); }
+        }
+
         /// <summary>
         /// Gets the root of the popup window.
         /// </summary>
diff --git a/src/Avalonia.Controls/Primitives/ScrollBar.cs b/src/Avalonia.Controls/Primitives/ScrollBar.cs
index 0057b15150..3ddcb06303 100644
--- a/src/Avalonia.Controls/Primitives/ScrollBar.cs
+++ b/src/Avalonia.Controls/Primitives/ScrollBar.cs
@@ -6,9 +6,21 @@ using System.Reactive;
 using System.Reactive.Linq;
 using Avalonia.Data;
 using Avalonia.Interactivity;
+using Avalonia.Input;
 
 namespace Avalonia.Controls.Primitives
 {
+    public class ScrollEventArgs : EventArgs
+    {
+        public ScrollEventArgs(ScrollEventType eventType, double newValue)
+        {
+            ScrollEventType = eventType;
+            NewValue = newValue;
+        }
+        public double NewValue { get; private set; }
+        public ScrollEventType ScrollEventType { get; private set; }
+    }
+
     /// <summary>
     /// A scrollbar control.
     /// </summary>
@@ -44,6 +56,9 @@ namespace Avalonia.Controls.Primitives
         {
             PseudoClass(OrientationProperty, o => o == Orientation.Vertical, ":vertical");
             PseudoClass(OrientationProperty, o => o == Orientation.Horizontal, ":horizontal");
+
+            Thumb.DragDeltaEvent.AddClassHandler<ScrollBar>(o => o.OnThumbDragDelta, RoutingStrategies.Bubble);
+            Thumb.DragCompletedEvent.AddClassHandler<ScrollBar>(o => o.OnThumbDragComplete, RoutingStrategies.Bubble);
         }
 
         /// <summary>
@@ -88,6 +103,8 @@ namespace Avalonia.Controls.Primitives
             set { SetValue(OrientationProperty, value); }
         }
 
+        public event EventHandler<ScrollEventArgs> Scroll;
+
         /// <summary>
         /// Calculates whether the scrollbar should be visible.
         /// </summary>
@@ -140,6 +157,8 @@ namespace Avalonia.Controls.Primitives
             _pageUpButton = e.NameScope.Find<Button>("PART_PageUpButton");
             _pageDownButton = e.NameScope.Find<Button>("PART_PageDownButton");
 
+
+
             if (_lineUpButton != null)
             {
                 _lineUpButton.Click += LineUpClick;
@@ -184,21 +203,39 @@ namespace Avalonia.Controls.Primitives
         private void SmallDecrement()
         {
             Value = Math.Max(Value - SmallChange * ViewportSize, Minimum);
+            OnScroll(ScrollEventType.SmallDecrement);
         }
 
         private void SmallIncrement()
         {
             Value = Math.Min(Value + SmallChange * ViewportSize, Maximum);
+            OnScroll(ScrollEventType.SmallIncrement);
         }
 
         private void LargeDecrement()
         {
             Value = Math.Max(Value - LargeChange * ViewportSize, Minimum);
+            OnScroll(ScrollEventType.LargeDecrement);
         }
 
         private void LargeIncrement()
         {
             Value = Math.Min(Value + LargeChange * ViewportSize, Maximum);
+            OnScroll(ScrollEventType.LargeIncrement);
+        }
+
+        private void OnThumbDragDelta(VectorEventArgs e)
+        {
+            OnScroll(ScrollEventType.ThumbTrack);
+        }
+        private void OnThumbDragComplete(VectorEventArgs e)
+        {
+            OnScroll(ScrollEventType.EndScroll);
+        }
+
+        protected void OnScroll(ScrollEventType scrollEventType)
+        {
+            Scroll?.Invoke(this, new ScrollEventArgs(scrollEventType, Value));
         }
     }
 }
diff --git a/src/Avalonia.Controls/Primitives/ScrollEventType.cs b/src/Avalonia.Controls/Primitives/ScrollEventType.cs
new file mode 100644
index 0000000000..ea639d82e0
--- /dev/null
+++ b/src/Avalonia.Controls/Primitives/ScrollEventType.cs
@@ -0,0 +1,53 @@
+// Copyright (c) The Avalonia Project. All rights reserved.
+// Licensed under the MIT license. See licence.md file in the project root for full license information.
+
+namespace Avalonia.Controls.Primitives
+{
+    /// <summary>    
+    /// Specifies the type of Avalonia.Controls.Primitives.ScrollBar.Scroll event
+    /// that occurred.
+    /// </summary>
+    public enum ScrollEventType
+    {
+        /// <summary>    
+        /// Specifies that the Avalonia.Controls.Primitives.Thumb moved a specified
+        /// distance, as determined by the value of Avalonia.Controls.Primitives.RangeBase.SmallChange.
+        /// The Avalonia.Controls.Primitives.Thumb moved to the left for a horizontal
+        /// Avalonia.Controls.Primitives.ScrollBar or upward for a vertical Avalonia.Controls.Primitives.ScrollBar.
+        /// </summary>
+        SmallDecrement = 0,
+        /// <summary>    
+        /// Specifies that the Avalonia.Controls.Primitives.Thumb moved a specified
+        /// distance, as determined by the value of Avalonia.Controls.Primitives.RangeBase.SmallChange.
+        /// The Avalonia.Controls.Primitives.Thumb moved to the right for a horizontal
+        /// Avalonia.Controls.Primitives.ScrollBar or downward for a vertical Avalonia.Controls.Primitives.ScrollBar.
+        /// </summary>
+        SmallIncrement = 1,
+        /// <summary>    
+        /// Specifies that the Avalonia.Controls.Primitives.Thumb moved a specified
+        /// distance, as determined by the value of Avalonia.Controls.Primitives.RangeBase.LargeChange.
+        /// The Avalonia.Controls.Primitives.Thumb moved to the left for a horizontal
+        /// Avalonia.Controls.Primitives.ScrollBar or upward for a vertical Avalonia.Controls.Primitives.ScrollBar.
+        /// </summary>
+        LargeDecrement = 2,
+        /// <summary>    
+        /// Specifies that the Avalonia.Controls.Primitives.Thumb moved a specified
+        /// distance, as determined by the value of Avalonia.Controls.Primitives.RangeBase.LargeChange.
+        /// The Avalonia.Controls.Primitives.Thumb moved to the right for a horizontal
+        /// Avalonia.Controls.Primitives.ScrollBar or downward for a vertical Avalonia.Controls.Primitives.ScrollBar.
+        /// </summary>
+        LargeIncrement = 3,
+        /// <summary>    
+        /// The Avalonia.Controls.Primitives.Thumb was dragged and caused a Avalonia.UIElement.MouseMove
+        /// event. A Avalonia.Controls.Primitives.ScrollBar.Scroll event of this Avalonia.Controls.Primitives.ScrollEventType
+        /// may occur more than one time when the Avalonia.Controls.Primitives.Thumb
+        /// is dragged in the Avalonia.Controls.Primitives.ScrollBar.
+        /// </summary>
+        ThumbTrack = 4,
+        /// <summary>    
+        /// Specifies that the Avalonia.Controls.Primitives.Thumb was dragged to a
+        /// new position and is now no longer being dragged by the user.
+        /// </summary>
+        EndScroll = 5
+    }
+}
diff --git a/src/Avalonia.Controls/ToolTip.cs b/src/Avalonia.Controls/ToolTip.cs
index 464e4188d2..10e964d014 100644
--- a/src/Avalonia.Controls/ToolTip.cs
+++ b/src/Avalonia.Controls/ToolTip.cs
@@ -234,11 +234,12 @@ namespace Avalonia.Controls
         {
             Close();
 
-            _popup = new PopupRoot { Content = this };
+            _popup = new PopupRoot { Content = this,  };
             ((ISetLogicalParent)_popup).SetParent(control);
             _popup.Position = Popup.GetPosition(control, GetPlacement(control), _popup,
                 GetHorizontalOffset(control), GetVerticalOffset(control));
             _popup.Show();
+            _popup.SnapInsideScreenEdges();
         }
 
         private void Close()
diff --git a/src/Avalonia.Controls/WindowBase.cs b/src/Avalonia.Controls/WindowBase.cs
index c427df1c26..5d66bee2f8 100644
--- a/src/Avalonia.Controls/WindowBase.cs
+++ b/src/Avalonia.Controls/WindowBase.cs
@@ -38,6 +38,9 @@ namespace Avalonia.Controls
                 o => o.Owner,
                 (o, v) => o.Owner = v);
 
+        public static readonly StyledProperty<bool> TopmostProperty =
+            AvaloniaProperty.Register<WindowBase, bool>(nameof(Topmost));
+
         private bool _hasExecutedInitialLayoutPass;
         private bool _isActive;
         private bool _ignoreVisibilityChange;
@@ -52,6 +55,8 @@ namespace Avalonia.Controls
             MinHeightProperty.Changed.AddClassHandler<WindowBase>((w, e) => w.PlatformImpl?.SetMinMaxSize(new Size(w.MinWidth, (double)e.NewValue), new Size(w.MaxWidth, w.MaxHeight)));
             MaxWidthProperty.Changed.AddClassHandler<WindowBase>((w, e) => w.PlatformImpl?.SetMinMaxSize(new Size(w.MinWidth, w.MinHeight), new Size((double)e.NewValue, w.MaxHeight)));
             MaxHeightProperty.Changed.AddClassHandler<WindowBase>((w, e) => w.PlatformImpl?.SetMinMaxSize(new Size(w.MinWidth, w.MinHeight), new Size(w.MaxWidth, (double)e.NewValue)));
+            
+            TopmostProperty.Changed.AddClassHandler<WindowBase>((w, e) => w.PlatformImpl?.SetTopmost((bool)e.NewValue));
         }
 
         public WindowBase(IWindowBaseImpl impl) : this(impl, AvaloniaLocator.Current)
@@ -124,6 +129,15 @@ namespace Avalonia.Controls
             set { SetAndRaise(OwnerProperty, ref _owner, value); }
         }
 
+        /// <summary>
+        /// Gets or sets whether this window appears on top of all other windows
+        /// </summary>
+        public bool Topmost
+        {
+            get { return GetValue(TopmostProperty); }
+            set { SetValue(TopmostProperty, value); }
+        }
+
         /// <summary>
         /// Activates the window.
         /// </summary>
diff --git a/src/Avalonia.DesignerSupport/Remote/PreviewerWindowImpl.cs b/src/Avalonia.DesignerSupport/Remote/PreviewerWindowImpl.cs
index ef16d06b60..d9a30f0d29 100644
--- a/src/Avalonia.DesignerSupport/Remote/PreviewerWindowImpl.cs
+++ b/src/Avalonia.DesignerSupport/Remote/PreviewerWindowImpl.cs
@@ -102,5 +102,9 @@ namespace Avalonia.DesignerSupport.Remote
         public void CanResize(bool value)
         {
         }
+
+        public void SetTopmost(bool value)
+        {
+        }
     }
 }
\ No newline at end of file
diff --git a/src/Avalonia.DesignerSupport/Remote/Stubs.cs b/src/Avalonia.DesignerSupport/Remote/Stubs.cs
index e749d10468..43f0e0a95e 100644
--- a/src/Avalonia.DesignerSupport/Remote/Stubs.cs
+++ b/src/Avalonia.DesignerSupport/Remote/Stubs.cs
@@ -104,6 +104,10 @@ namespace Avalonia.DesignerSupport.Remote
         public void CanResize(bool value)
         {
         }
+
+        public void SetTopmost(bool value)
+        {
+        }
     }
 
     class ClipboardStub : IClipboard
diff --git a/src/Avalonia.Themes.Default/Calendar.xaml b/src/Avalonia.Themes.Default/Calendar.xaml
index 5e4306c217..61b1224424 100644
--- a/src/Avalonia.Themes.Default/Calendar.xaml
+++ b/src/Avalonia.Themes.Default/Calendar.xaml
@@ -15,9 +15,10 @@
     <Setter Property="Template">
       <ControlTemplate>
         <StackPanel Name="Root"
-                    HorizontalAlignment="Center">
+                    HorizontalAlignment="Center"
+                    ClipToBounds="True">
 
-          <CalendarItem Name="CalendarItem"
+                    <CalendarItem Name="CalendarItem"
                         Background="{TemplateBinding Background}"
                         BorderBrush="{TemplateBinding BorderBrush}"
                         BorderThickness="{TemplateBinding BorderThickness}"
diff --git a/src/Avalonia.Visuals/Media/FontFamily.cs b/src/Avalonia.Visuals/Media/FontFamily.cs
index a4b4f5085a..70a4aee4cd 100644
--- a/src/Avalonia.Visuals/Media/FontFamily.cs
+++ b/src/Avalonia.Visuals/Media/FontFamily.cs
@@ -77,10 +77,10 @@ namespace Avalonia.Media
         /// <summary>
         /// Implicit conversion of string to FontFamily
         /// </summary>
-        /// <param name="fontFamily"></param>
-        public static implicit operator FontFamily(string fontFamily)
+        /// <param name="s"></param>
+        public static implicit operator FontFamily(string s)
         {
-            return new FontFamily(fontFamily);
+            return Parse(s);
         }
 
         /// <summary>
diff --git a/src/Gtk/Avalonia.Gtk3/Interop/Native.cs b/src/Gtk/Avalonia.Gtk3/Interop/Native.cs
index 1adaf9f4e1..9cc6ba6901 100644
--- a/src/Gtk/Avalonia.Gtk3/Interop/Native.cs
+++ b/src/Gtk/Avalonia.Gtk3/Interop/Native.cs
@@ -261,10 +261,13 @@ namespace Avalonia.Gtk3.Interop
 
             [UnmanagedFunctionPointer(CallingConvention.Cdecl), GtkImport(GtkDll.Gtk)]
             public delegate void gtk_window_unmaximize(GtkWindow window);
-            
+
             [UnmanagedFunctionPointer(CallingConvention.Cdecl), GtkImport(GtkDll.Gtk)]
             public delegate void gtk_window_close(GtkWindow window);
 
+            [UnmanagedFunctionPointer(CallingConvention.Cdecl), GtkImport(GtkDll.Gtk)]
+            public delegate void gtk_window_set_keep_above(GtkWindow gtkWindow, bool setting);
+
             [UnmanagedFunctionPointer(CallingConvention.Cdecl), GtkImport(GtkDll.Gtk)]
             public delegate void gtk_window_set_geometry_hints(GtkWindow window, IntPtr geometry_widget, ref GdkGeometry geometry, GdkWindowHints geom_mask);
 
@@ -472,6 +475,7 @@ namespace Avalonia.Gtk3.Interop
         public static D.gtk_window_maximize GtkWindowMaximize;
         public static D.gtk_window_unmaximize GtkWindowUnmaximize;
         public static D.gtk_window_close GtkWindowClose;
+        public static D.gtk_window_set_keep_above GtkWindowSetKeepAbove;
         public static D.gdk_window_begin_move_drag GdkWindowBeginMoveDrag;
         public static D.gdk_window_begin_resize_drag GdkWindowBeginResizeDrag;
         public static D.gdk_event_request_motions GdkEventRequestMotions;
diff --git a/src/Gtk/Avalonia.Gtk3/WindowBaseImpl.cs b/src/Gtk/Avalonia.Gtk3/WindowBaseImpl.cs
index 94537d3475..8a880fd306 100644
--- a/src/Gtk/Avalonia.Gtk3/WindowBaseImpl.cs
+++ b/src/Gtk/Avalonia.Gtk3/WindowBaseImpl.cs
@@ -416,6 +416,8 @@ namespace Avalonia.Gtk3
 
         public void Hide() => Native.GtkWidgetHide(GtkWidget);
 
+        public void SetTopmost(bool value) => Native.GtkWindowSetKeepAbove(GtkWidget, value);
+
         void GetGlobalPointer(out int x, out int y)
         {
             int mask;
diff --git a/src/Gtk/Avalonia.Gtk3/WindowImpl.cs b/src/Gtk/Avalonia.Gtk3/WindowImpl.cs
index bae34db6f3..a0b754c229 100644
--- a/src/Gtk/Avalonia.Gtk3/WindowImpl.cs
+++ b/src/Gtk/Avalonia.Gtk3/WindowImpl.cs
@@ -81,7 +81,7 @@ namespace Avalonia.Gtk3
         public void ShowTaskbarIcon(bool value) => Native.GtkWindowSetSkipTaskbarHint(GtkWidget, !value);
 
         public void CanResize(bool value) => Native.GtkWindowSetResizable(GtkWidget, value);
-        
+
 
         class EmptyDisposable : IDisposable
         {
diff --git a/src/Markup/Avalonia.Markup/Data/MultiBinding.cs b/src/Markup/Avalonia.Markup/Data/MultiBinding.cs
index f6f62dcd1d..a3fa6880ac 100644
--- a/src/Markup/Avalonia.Markup/Data/MultiBinding.cs
+++ b/src/Markup/Avalonia.Markup/Data/MultiBinding.cs
@@ -10,6 +10,7 @@ using System.Reactive.Subjects;
 using Avalonia.Controls;
 using Avalonia.Data.Converters;
 using Avalonia.Metadata;
+using JetBrains.Annotations;
 
 namespace Avalonia.Data
 {
@@ -65,7 +66,7 @@ namespace Avalonia.Data
             var children = Bindings.Select(x => x.Initiate(target, null));
             var input = children.Select(x => x.Subject).CombineLatest().Select(x => ConvertValue(x, targetType));
             var mode = Mode == BindingMode.Default ?
-                targetProperty.GetMetadata(target.GetType()).DefaultBindingMode : Mode;
+                targetProperty?.GetMetadata(target.GetType()).DefaultBindingMode : Mode;
 
             switch (mode)
             {
diff --git a/src/OSX/Avalonia.MonoMac/WindowBaseImpl.cs b/src/OSX/Avalonia.MonoMac/WindowBaseImpl.cs
index 8cbc6cbdd8..89cef59b53 100644
--- a/src/OSX/Avalonia.MonoMac/WindowBaseImpl.cs
+++ b/src/OSX/Avalonia.MonoMac/WindowBaseImpl.cs
@@ -123,6 +123,7 @@ namespace Avalonia.MonoMac
 
         public void Hide() => Window?.OrderOut(Window);
 
+        public void SetTopmost(bool value) => Window.Level = value ? NSWindowLevel.Floating : NSWindowLevel.Normal;
 
         public void BeginMoveDrag()
         {
diff --git a/src/Skia/Avalonia.Skia/FormattedTextImpl.cs b/src/Skia/Avalonia.Skia/FormattedTextImpl.cs
index 00f0a48a7b..13dcd9669d 100644
--- a/src/Skia/Avalonia.Skia/FormattedTextImpl.cs
+++ b/src/Skia/Avalonia.Skia/FormattedTextImpl.cs
@@ -560,13 +560,11 @@ namespace Avalonia.Skia
                 }
 
                 measured = LineBreak(Text, curOff, length, _paint, constraint, out trailingnumber);
-
                 AvaloniaFormattedTextLine line = new AvaloniaFormattedTextLine();
                 line.TextLength = measured;
-
+                line.Start = curOff;
                 subString = Text.Substring(line.Start, line.TextLength);
                 lineWidth = _paint.MeasureText(subString);
-                line.Start = curOff;
                 line.Length = measured - trailingnumber;
                 line.Width = lineWidth;
                 line.Height = _lineHeight;
@@ -575,10 +573,33 @@ namespace Avalonia.Skia
                 _skiaLines.Add(line);
 
                 curY += _lineHeight;
-
                 curY += mLeading;
-
                 curOff += measured;
+
+                //if this is the last line and there are trailing newline characters then
+                //insert a additional line
+                if (curOff >= length)
+                {
+                    var subStringMinusNewlines = subString.TrimEnd('\n', '\r');
+                    var lengthDiff = subString.Length - subStringMinusNewlines.Length;
+                    if (lengthDiff > 0)
+                    {
+                        AvaloniaFormattedTextLine lastLine = new AvaloniaFormattedTextLine();
+                        lastLine.TextLength = lengthDiff;
+                        lastLine.Start = curOff - lengthDiff;
+                        var lastLineSubString = Text.Substring(line.Start, line.TextLength);
+                        var lastLineWidth = _paint.MeasureText(lastLineSubString);
+                        lastLine.Length = 0;
+                        lastLine.Width = lastLineWidth;
+                        lastLine.Height = _lineHeight;
+                        lastLine.Top = curY;
+
+                        _skiaLines.Add(lastLine);
+
+                        curY += _lineHeight;
+                        curY += mLeading;
+                    }
+                }
             }
 
             // Now convert to Avalonia data formats
diff --git a/src/Windows/Avalonia.Win32/Interop/UnmanagedMethods.cs b/src/Windows/Avalonia.Win32/Interop/UnmanagedMethods.cs
index 86dcec410b..daaee9636e 100644
--- a/src/Windows/Avalonia.Win32/Interop/UnmanagedMethods.cs
+++ b/src/Windows/Avalonia.Win32/Interop/UnmanagedMethods.cs
@@ -78,6 +78,14 @@ namespace Avalonia.Win32.Interop
             SWP_RESIZE = SWP_NOACTIVATE | SWP_NOMOVE | SWP_NOZORDER
         }
 
+        public static class WindowPosZOrder
+        {
+            public static readonly IntPtr HWND_BOTTOM = new IntPtr(1);
+            public static readonly IntPtr HWND_TOP = new IntPtr(0);
+            public static readonly IntPtr HWND_TOPMOST = new IntPtr(-1);
+            public static readonly IntPtr HWND_NOTOPMOST = new IntPtr(-2);
+        }
+
         public enum SizeCommand
         {
             Restored,
diff --git a/src/Windows/Avalonia.Win32/SystemDialogImpl.cs b/src/Windows/Avalonia.Win32/SystemDialogImpl.cs
index e08fecbbd9..9b6bf57e92 100644
--- a/src/Windows/Avalonia.Win32/SystemDialogImpl.cs
+++ b/src/Windows/Avalonia.Win32/SystemDialogImpl.cs
@@ -54,7 +54,7 @@ namespace Avalonia.Win32
                 var fileBuffer = new char[256];
                 dialog.InitialFileName?.CopyTo(0, fileBuffer, 0, dialog.InitialFileName.Length);
 
-                string userSelectedExt = null;
+                string userSelectedExt = string.Empty;
 
 
                 var title = ToChars(dialog.Title);
diff --git a/src/Windows/Avalonia.Win32/WindowImpl.cs b/src/Windows/Avalonia.Win32/WindowImpl.cs
index 159c8386b6..9f67f97252 100644
--- a/src/Windows/Avalonia.Win32/WindowImpl.cs
+++ b/src/Windows/Avalonia.Win32/WindowImpl.cs
@@ -32,6 +32,7 @@ namespace Avalonia.Win32
         private bool _trackingMouse;
         private bool _decorated = true;
         private bool _resizable = true;
+        private bool _topmost = false;
         private double _scaling = 1;
         private WindowState _showWindowState;
         private WindowState _lastWindowState;
@@ -841,7 +842,7 @@ namespace Avalonia.Win32
                     var cx = Math.Abs(monitorInfo.rcWork.right - x);
                     var cy = Math.Abs(monitorInfo.rcWork.bottom - y);
 
-                    SetWindowPos(_hwnd, new IntPtr(-2), x, y, cx, cy, SetWindowPosFlags.SWP_SHOWWINDOW);
+                    SetWindowPos(_hwnd, WindowPosZOrder.HWND_NOTOPMOST, x, y, cx, cy, SetWindowPosFlags.SWP_SHOWWINDOW);
                 }
             }
         }
@@ -904,5 +905,21 @@ namespace Avalonia.Win32
 
             _resizable = value;
         }
+
+        public void SetTopmost(bool value)
+        {
+            if (value == _topmost)
+            {
+                return;
+            }
+
+            IntPtr hWndInsertAfter = value ? WindowPosZOrder.HWND_TOPMOST : WindowPosZOrder.HWND_NOTOPMOST;
+            UnmanagedMethods.SetWindowPos(_hwnd,
+                   hWndInsertAfter,
+                   0, 0, 0, 0,
+                   SetWindowPosFlags.SWP_NOMOVE | SetWindowPosFlags.SWP_NOSIZE | SetWindowPosFlags.SWP_NOACTIVATE);
+
+            _topmost = value;
+        }
     }
 }
diff --git a/tests/Avalonia.Base.UnitTests/AvaloniaObjectTests_Binding.cs b/tests/Avalonia.Base.UnitTests/AvaloniaObjectTests_Binding.cs
index 02fb1f11ad..4638aa84a5 100644
--- a/tests/Avalonia.Base.UnitTests/AvaloniaObjectTests_Binding.cs
+++ b/tests/Avalonia.Base.UnitTests/AvaloniaObjectTests_Binding.cs
@@ -457,6 +457,28 @@ namespace Avalonia.Base.UnitTests
             Assert.True(target.IsAnimating(Class1.FooProperty));
         }
 
+        [Fact]
+        public void TwoWay_Binding_Should_Not_Call_Setter_On_Creation()
+        {
+            var target = new Class1();
+            var source = new TestTwoWayBindingViewModel();
+
+            target.Bind(Class1.DoubleValueProperty, new Binding(nameof(source.Value), BindingMode.TwoWay) { Source = source });
+
+            Assert.False(source.SetterCalled);
+        }
+
+        [Fact]
+        public void TwoWay_Binding_Should_Not_Call_Setter_On_Creation_Indexer()
+        {
+            var target = new Class1();
+            var source = new TestTwoWayBindingViewModel();
+
+            target.Bind(Class1.DoubleValueProperty, new Binding("[0]", BindingMode.TwoWay) { Source = source });
+
+            Assert.False(source.SetterCalled);
+        }
+
         /// <summary>
         /// Returns an observable that returns a single value but does not complete.
         /// </summary>
@@ -545,5 +567,32 @@ namespace Avalonia.Base.UnitTests
                 }
             }
         }
+
+        private class TestTwoWayBindingViewModel
+        {
+            private double _value;
+
+            public double Value
+            {
+                get => _value;
+                set
+                {
+                    _value = value;
+                    SetterCalled = true;
+                }
+            }
+
+            public double this[int index]
+            {
+                get => _value;
+                set
+                {
+                    _value = value;
+                    SetterCalled = true;
+                }
+            }
+
+            public bool SetterCalled { get; private set; }
+        }
     }
 }
\ No newline at end of file
diff --git a/tests/Avalonia.Controls.UnitTests/CarouselTests.cs b/tests/Avalonia.Controls.UnitTests/CarouselTests.cs
index df61698209..f36e5864f6 100644
--- a/tests/Avalonia.Controls.UnitTests/CarouselTests.cs
+++ b/tests/Avalonia.Controls.UnitTests/CarouselTests.cs
@@ -170,6 +170,41 @@ namespace Avalonia.Controls.UnitTests
             Assert.Equal("Bar", ((TextBlock)child).Text);
         }
 
+        [Fact]
+        public void Selected_Index_Changes_To_When_Items_Assigned_Null()
+        {
+            var items = new ObservableCollection<string>
+            {
+               "Foo",
+               "Bar",
+               "FooBar"
+            };
+
+            var target = new Carousel
+            {
+                Template = new FuncControlTemplate<Carousel>(CreateTemplate),
+                Items = items,
+                IsVirtualized = false
+            };
+
+            target.ApplyTemplate();
+            target.Presenter.ApplyTemplate();
+
+            Assert.Single(target.GetLogicalChildren());
+
+            var child = target.GetLogicalChildren().Single();
+
+            Assert.IsType<TextBlock>(child);
+            Assert.Equal("Foo", ((TextBlock)child).Text);
+
+            target.Items = null;
+
+            var numChildren = target.GetLogicalChildren().Count();
+
+            Assert.Equal(0, numChildren);
+            Assert.Equal(-1, target.SelectedIndex);
+        }
+
         [Fact]
         public void Selected_Index_Is_Maintained_Carousel_Created_With_Non_Zero_SelectedIndex()
         {
diff --git a/tests/Avalonia.Controls.UnitTests/Primitives/ScrollBarTests.cs b/tests/Avalonia.Controls.UnitTests/Primitives/ScrollBarTests.cs
index 0af4d791c7..672b5c608b 100644
--- a/tests/Avalonia.Controls.UnitTests/Primitives/ScrollBarTests.cs
+++ b/tests/Avalonia.Controls.UnitTests/Primitives/ScrollBarTests.cs
@@ -5,6 +5,7 @@ using System;
 using System.Linq;
 using Avalonia.Controls.Primitives;
 using Avalonia.Controls.Templates;
+using Avalonia.Input;
 using Avalonia.Media;
 using Xunit;
 
@@ -59,6 +60,64 @@ namespace Avalonia.Controls.UnitTests.Primitives
             Assert.Equal(50, target.Value);
         }
 
+        [Fact]
+        public void Thumb_DragDelta_Event_Should_Raise_Scroll_Event()
+        {
+            var target = new ScrollBar
+            {
+                Template = new FuncControlTemplate<ScrollBar>(Template),
+            };
+
+            target.ApplyTemplate();
+
+            var track = (Track)target.GetTemplateChildren().First(x => x.Name == "track");
+
+            var raisedEvent = Assert.Raises<ScrollEventArgs>(
+                handler => target.Scroll += handler,
+                handler => target.Scroll -= handler,
+                () =>
+                {
+                    var ev = new VectorEventArgs
+                    {
+                        RoutedEvent = Thumb.DragDeltaEvent,
+                        Vector = new Vector(0, 0)
+                    };
+
+                    track.Thumb.RaiseEvent(ev);
+                });
+
+            Assert.Equal(ScrollEventType.ThumbTrack, raisedEvent.Arguments.ScrollEventType);
+        }
+
+        [Fact]
+        public void Thumb_DragComplete_Event_Should_Raise_Scroll_Event()
+        {
+            var target = new ScrollBar
+            {
+                Template = new FuncControlTemplate<ScrollBar>(Template),
+            };
+
+            target.ApplyTemplate();
+
+            var track = (Track)target.GetTemplateChildren().First(x => x.Name == "track");
+
+            var raisedEvent = Assert.Raises<ScrollEventArgs>(
+                handler => target.Scroll += handler,
+                handler => target.Scroll -= handler,
+                () =>
+                {
+                    var ev = new VectorEventArgs
+                    {
+                        RoutedEvent = Thumb.DragCompletedEvent,
+                        Vector = new Vector(0, 0)
+                    };
+
+                    track.Thumb.RaiseEvent(ev);
+                });
+
+            Assert.Equal(ScrollEventType.EndScroll, raisedEvent.Arguments.ScrollEventType);
+        }
+
         [Fact]
         public void ScrollBar_Can_AutoHide()
         {