[Popup] opens under mouse as 0.8 release did. Prevents rapid opening and closing.

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

@@ -305,7 +305,7 @@ namespace Avalonia.Controls.Primitives.PopupPositioning
             positionerParameters.ConstraintAdjustment = PopupPositionerConstraintAdjustment.All;
             if (placement == PlacementMode.Pointer)
             {
-                positionerParameters.AnchorRectangle = new Rect(pointer, new Size(1, 1));
+                positionerParameters.AnchorRectangle = new Rect(pointer, new Size(0, 0));
                 positionerParameters.Anchor = PopupPositioningEdge.BottomRight;
                 positionerParameters.Gravity = PopupPositioningEdge.BottomRight;
             }