Font scaling on linux, and FolderBrowse implementation for linux.

samples/XamlTestApplicationPcl/ViewModels/MainWindowViewModel.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using ReactiveUI;
+using Perspex.Controls;
 
 namespace XamlTestApplication.ViewModels
 {
@@ -59,6 +60,23 @@ namespace XamlTestApplication.ViewModels
             CollapseNodesCommand.Subscribe(_ => ExpandNodes(false));
             ExpandNodesCommand = ReactiveCommand.Create();
             ExpandNodesCommand.Subscribe(_ => ExpandNodes(true));
+
+            OpenFileCommand = ReactiveCommand.Create();
+            OpenFileCommand.Subscribe(_ =>
+            {
+                var ofd = new OpenFileDialog();
+
+                ofd.ShowAsync();
+            });
+
+            OpenFolderCommand = ReactiveCommand.Create();
+            OpenFolderCommand.Subscribe(_ =>
+            {
+                var ofd = new OpenFolderDialog();
+
+                ofd.ShowAsync();
+            });
+
         }
 
         public List<TestItem> Items { get; }
@@ -68,6 +86,10 @@ namespace XamlTestApplication.ViewModels
 
         public ReactiveCommand<object> ExpandNodesCommand { get; }
 
+        public ReactiveCommand<object> OpenFileCommand { get; }
+
+        public ReactiveCommand<object> OpenFolderCommand { get; }
+
         public void ExpandNodes(bool expanded)
         {
             foreach (var node in Nodes)

samples/XamlTestApplicationPcl/Views/MainWindow.paml

@@ -6,11 +6,13 @@
         Title="Perspex Test Application" Width="800" Height="600">
   <Grid RowDefinitions="Auto,*,Auto" ColumnDefinitions="*,*">
     <Menu Grid.ColumnSpan="2">
-      <MenuItem Header="_File" Background="Red">
+      <MenuItem Header="_File">
+        <MenuItem Header="_Open" Command="{Binding OpenFileCommand}" />
+        <MenuItem Header="_Open Folder" Command="{Binding OpenFolderCommand}" />
         <MenuItem Header="_Hello">
           <MenuItem Header="_Goodbye"/>
           <Separator/>
-          <MenuItem Header="_World" Background="Red"/>
+          <MenuItem Header="_World" />
         </MenuItem>
         <Separator Background="Red"/>
         <MenuItem Header="_Test"/>

src/Gtk/Perspex.Cairo/Media/FormattedTextImpl.cs

@@ -15,6 +15,11 @@ namespace Perspex.Cairo.Media
         private Size _size;
         private readonly string _text;
 
+        static double CorrectScale(double input)
+        {
+            return input * 0.75;
+        }
+
         public FormattedTextImpl(
             Pango.Context context,
             string text,
@@ -25,14 +30,14 @@ namespace Perspex.Cairo.Media
             FontWeight fontWeight)
         {
             Contract.Requires<ArgumentNullException>(context != null);
-            Contract.Requires<ArgumentNullException> (text != null);
+            Contract.Requires<ArgumentNullException>(text != null);
             Layout = new Pango.Layout(context);
             _text = text;
             Layout.SetText(text);
             Layout.FontDescription = new Pango.FontDescription
             {
                 Family = fontFamily,
-                Size = Pango.Units.FromDouble(fontSize),
+                Size = Pango.Units.FromDouble(CorrectScale(fontSize)),
                 Style = (Pango.Style)fontStyle,
                 Weight = fontWeight.ToCairo()
             };

src/Gtk/Perspex.Gtk/SystemDialogImpl.cs

@@ -15,7 +15,7 @@ namespace Perspex.Gtk
         public Task<string[]> ShowFileDialogAsync(FileDialog dialog, IWindowImpl parent)
         {
             var tcs = new TaskCompletionSource<string[]>();
-            var dlg = new global::Gtk.FileChooserDialog(dialog.Title, ((WindowImpl) parent),
+            var dlg = new global::Gtk.FileChooserDialog(dialog.Title, ((WindowImpl)parent),
                 dialog is OpenFileDialog
                     ? FileChooserAction.Open
                     : FileChooserAction.Save,
@@ -44,7 +44,7 @@ namespace Perspex.Gtk
                 dlg.Hide();
                 dlg.Dispose();
             };
-            
+
             dlg.Close += delegate
             {
                 tcs.TrySetResult(null);
@@ -56,7 +56,33 @@ namespace Perspex.Gtk
 
         public Task<string> ShowFolderDialogAsync(OpenFolderDialog dialog, IWindowImpl parent)
         {
-            throw new NotImplementedException();
+            var tcs = new TaskCompletionSource<string>();
+            var dlg = new global::Gtk.FileChooserDialog(dialog.Title, ((WindowImpl)parent),
+                FileChooserAction.SelectFolder,
+                "Cancel", ResponseType.Cancel,
+                "Select Folder", ResponseType.Accept)
+            {
+
+            };
+
+            dlg.Modal = true;
+
+            dlg.Response += (_, args) =>
+            {
+                if (args.ResponseId == ResponseType.Accept)
+                    tcs.TrySetResult(dlg.Filename);
+
+                dlg.Hide();
+                dlg.Dispose();
+            };
+
+            dlg.Close += delegate
+            {
+                tcs.TrySetResult(null);
+                dlg.Dispose();
+            };
+            dlg.Show();
+            return tcs.Task;
         }
     }
 }

src/Perspex.Controls/Utils/UndoRedoHelper.cs

@@ -37,7 +37,12 @@ namespace Perspex.Controls.Utils
 
         public void Undo()
         {
-            _host.UndoRedoState= (_currentNode = _currentNode?.Previous ?? _currentNode).Value;
+			if (_currentNode != null)  
+			{
+				_currentNode = _currentNode.Previous;
+			}
+
+			_host.UndoRedoState = _currentNode.Value;            
         }
 
         public bool IsLastState => _currentNode.Next == null;
@@ -62,8 +67,12 @@ namespace Perspex.Controls.Utils
         }
 
         public void Redo()
-        {
+        {            
-            _host.UndoRedoState = (_currentNode = _currentNode?.Next ?? _currentNode).Value;
+			if (_currentNode != null) {
+				_currentNode = _currentNode.Next;
+			}
+
+			_host.UndoRedoState = _currentNode.Value;
         }
 
         public void Snapshot()