Fix scrolling for widgets inside a Paned

Xwt.WPF/Xwt.WPFBackend/PanedBackend.cs

@@ -374,6 +374,20 @@ namespace Xwt.WPFBackend
 
 				panel1.Size = new GridLength (position, GridUnitType.Star);
 				panel2.Size = new GridLength (availableSize - position, GridUnitType.Star);
+
+				// The measure method has to be called on the children. Widgets like TreeView require
+				// a Measure call with the real size constraints, otherwise it can't decide if the
+				// scrollbars have to be shown or not
+				panel1.Widget.InvalidateMeasure ();
+				panel2.Widget.InvalidateMeasure ();
+				if (direction == Orientation.Horizontal) {
+					panel1.Widget.Measure (new SW.Size (position, size.Height));
+					panel2.Widget.Measure (new SW.Size (availableSize - position, size.Height));
+				}
+				else {
+					panel1.Widget.Measure (new SW.Size (size.Width, position));
+					panel2.Widget.Measure (new SW.Size (size.Width, availableSize - position));
+				}
 			}
 			else if (panel1.Widget != null)
 				panel1.Size = new GridLength (1, GridUnitType.Star);