Added PlacementConstraintAdjustment property.

To `Popup` and `ContextMenu`. Set default value to be `FlipX | FlipY` to try to match WPF's defaults.

src/Avalonia.Controls/ContextMenu.cs

@@ -20,7 +20,6 @@ namespace Avalonia.Controls
     /// </summary>
     public class ContextMenu : MenuBase, ISetterValue
     {
-
         /// <summary>
         /// Defines the <see cref="HorizontalOffset"/> property.
         /// </summary>
@@ -39,6 +38,12 @@ namespace Avalonia.Controls
         public static readonly StyledProperty<PopupAnchor> PlacementAnchorProperty =
             Popup.PlacementAnchorProperty.AddOwner<ContextMenu>();
 
+        /// <summary>
+        /// Defines the <see cref="PlacementConstraintAdjustment"/> property.
+        /// </summary>
+        public static readonly StyledProperty<PopupPositionerConstraintAdjustment> PlacementConstraintAdjustmentProperty =
+            Popup.PlacementConstraintAdjustmentProperty.AddOwner<ContextMenu>();
+
         /// <summary>
         /// Defines the <see cref="PlacementGravity"/> property.
         /// </summary>
@@ -124,6 +129,16 @@ namespace Avalonia.Controls
             set { SetValue(PlacementAnchorProperty, value); }
         }
 
+        /// <summary>
+        /// Gets or sets a value describing how the context menu position will be adjusted if the
+        /// unadjusted position would result in the context menu being partly constrained.
+        /// </summary>
+        public PopupPositionerConstraintAdjustment PlacementConstraintAdjustment
+        {
+            get { return GetValue(PlacementConstraintAdjustmentProperty); }
+            set { SetValue(PlacementConstraintAdjustmentProperty, value); }
+        }
+
         /// <summary>
         /// Gets or sets a value which defines in what direction the context menu should open
         /// when <see cref="PlacementMode"/> is <see cref="PlacementMode.AnchorAndGravity"/>.
@@ -245,6 +260,7 @@ namespace Avalonia.Controls
                     HorizontalOffset = HorizontalOffset,
                     VerticalOffset = VerticalOffset,
                     PlacementAnchor = PlacementAnchor,
+                    PlacementConstraintAdjustment = PlacementConstraintAdjustment,
                     PlacementGravity = PlacementGravity,
                     PlacementMode = PlacementMode,
                     PlacementRect = PlacementRect,

src/Avalonia.Controls/Primitives/IPopupHost.cs

@@ -52,6 +52,7 @@ namespace Avalonia.Controls.Primitives
         void ConfigurePosition(IVisual target, PlacementMode placement, Point offset,
             PopupAnchor anchor = PopupAnchor.None,
             PopupGravity gravity = PopupGravity.None,
+            PopupPositionerConstraintAdjustment constraintAdjustment = PopupPositionerConstraintAdjustment.All,
             Rect? rect = null);
 
         /// <summary>

src/Avalonia.Controls/Primitives/OverlayPopupHost.cs

@@ -72,10 +72,11 @@ namespace Avalonia.Controls.Primitives
 
         public void ConfigurePosition(IVisual target, PlacementMode placement, Point offset,
             PopupAnchor anchor = PopupAnchor.None, PopupGravity gravity = PopupGravity.None,
+            PopupPositionerConstraintAdjustment constraintAdjustment = PopupPositionerConstraintAdjustment.All,
             Rect? rect = null)
         {
             _positionerParameters.ConfigurePosition((TopLevel)_overlayLayer.GetVisualRoot(), target, placement, offset, anchor,
-                gravity, rect);
+                gravity, constraintAdjustment, rect);
             UpdatePosition();
         }
 

src/Avalonia.Controls/Primitives/Popup.cs

@@ -44,6 +44,14 @@ namespace Avalonia.Controls.Primitives
         public static readonly StyledProperty<PopupAnchor> PlacementAnchorProperty =
             AvaloniaProperty.Register<Popup, PopupAnchor>(nameof(PlacementAnchor));
 
+        /// <summary>
+        /// Defines the <see cref="PlacementConstraintAdjustment"/> property.
+        /// </summary>
+        public static readonly StyledProperty<PopupPositionerConstraintAdjustment> PlacementConstraintAdjustmentProperty =
+            AvaloniaProperty.Register<Popup, PopupPositionerConstraintAdjustment>(
+                nameof(PlacementConstraintAdjustment),
+                PopupPositionerConstraintAdjustment.FlipX | PopupPositionerConstraintAdjustment.FlipY);
+
         /// <summary>
         /// Defines the <see cref="PlacementGravity"/> property.
         /// </summary>
@@ -174,6 +182,16 @@ namespace Avalonia.Controls.Primitives
             set { SetValue(PlacementAnchorProperty, value); }
         }
 
+        /// <summary>
+        /// Gets or sets a value describing how the popup position will be adjusted if the
+        /// unadjusted position would result in the popup being partly constrained.
+        /// </summary>
+        public PopupPositionerConstraintAdjustment PlacementConstraintAdjustment
+        {
+            get { return GetValue(PlacementConstraintAdjustmentProperty); }
+            set { SetValue(PlacementConstraintAdjustmentProperty, value); }
+        }
+
         /// <summary>
         /// Gets or sets a value which defines in what direction the popup should open
         /// when <see cref="PlacementMode"/> is <see cref="PlacementMode.AnchorAndGravity"/>.
@@ -320,6 +338,7 @@ namespace Avalonia.Controls.Primitives
                 new Point(HorizontalOffset, VerticalOffset),
                 PlacementAnchor,
                 PlacementGravity,
+                PlacementConstraintAdjustment,
                 PlacementRect);
 
             DeferCleanup(SubscribeToEventHandler<IPopupHost, EventHandler<TemplateAppliedEventArgs>>(popupHost, RootTemplateApplied,

src/Avalonia.Controls/Primitives/PopupPositioning/IPopupPositioner.cs

@@ -428,13 +428,14 @@ namespace Avalonia.Controls.Primitives.PopupPositioning
         public static void ConfigurePosition(ref this PopupPositionerParameters positionerParameters,
             TopLevel topLevel,
             IVisual target, PlacementMode placement, Point offset,
-            PopupAnchor anchor, PopupGravity gravity, Rect? rect)
+            PopupAnchor anchor, PopupGravity gravity,
+            PopupPositionerConstraintAdjustment constraintAdjustment, Rect? rect)
         {
             // We need a better way for tracking the last pointer position
             var pointer = topLevel.PointToClient(topLevel.PlatformImpl.MouseDevice.Position);
             
             positionerParameters.Offset = offset;
-            positionerParameters.ConstraintAdjustment = PopupPositionerConstraintAdjustment.All;
+            positionerParameters.ConstraintAdjustment = constraintAdjustment;
             if (placement == PlacementMode.Pointer)
             {
                 positionerParameters.AnchorRectangle = new Rect(pointer, new Size(1, 1));

src/Avalonia.Controls/Primitives/PopupRoot.cs

@@ -84,10 +84,11 @@ namespace Avalonia.Controls.Primitives
         public void ConfigurePosition(IVisual target, PlacementMode placement, Point offset,
             PopupAnchor anchor = PopupAnchor.None,
             PopupGravity gravity = PopupGravity.None,
+            PopupPositionerConstraintAdjustment constraintAdjustment = PopupPositionerConstraintAdjustment.All,
             Rect? rect = null)
         {
             _positionerParameters.ConfigurePosition(_parent, target,
-                placement, offset, anchor, gravity, rect);
+                placement, offset, anchor, gravity, constraintAdjustment, rect);
 
             if (_positionerParameters.Size != default)
                 UpdatePosition();