Fix layout issue with ListBox and ListView

Xwt.WPF/Xwt.WPFBackend/BoxBackend.cs

@@ -105,8 +105,16 @@ namespace Xwt.WPFBackend
 			// Use the 'widgets' field so we can easily map a control position by looking at 'rects'.
 			for (int i = 0; i < widgets.Length; i++) {
 				var element = WidgetBackend.GetFrameworkElement (widgets [i]);
-				if (!element.IsArrangeValid || force)
-					element.Arrange (DataConverter.ToWpfRect (rects [i]));
+				if (!element.IsArrangeValid || force) {
+					// Measure the widget again using the allocation constraints. This is necessary
+					// because WPF widgets my cache some measurement information based on the
+					// constraints provided in the last Measure call (which when calculating the
+					// preferred size is normally set to infinite.
+					element.InvalidateMeasure ();
+					element.Measure (new SW.Size (rects[i].Width, rects[i].Height));
+
+					element.Arrange (DataConverter.ToWpfRect (rects[i]));
+				}
 			}
 		}
 	}

Xwt.WPF/Xwt.WPFBackend/ExListBox.cs

@@ -142,6 +142,12 @@ namespace Xwt.WPFBackend
 		protected override System.Windows.Size MeasureOverride (System.Windows.Size constraint)
 		{
 			var s = base.MeasureOverride (constraint);
+
+			if (ScrollViewer.GetHorizontalScrollBarVisibility (this) != ScrollBarVisibility.Hidden)
+				s.Width = 0;
+			if (ScrollViewer.GetVerticalScrollBarVisibility (this) != ScrollBarVisibility.Hidden)
+				s.Height = SystemParameters.CaptionHeight;
+
 			return Backend.MeasureOverride (constraint, s);
 		}
 	}

Xwt.WPF/Xwt.WPFBackend/ExListView.cs

@@ -49,11 +49,8 @@ namespace Xwt.WPFBackend
 
 		protected override System.Windows.Size MeasureOverride (System.Windows.Size constraint)
 		{
-			// This line is commented because it breaks listview scrolling
-			// We have to find a solution because the current implementation is not fully correct
-			// var s = base.MeasureOverride (constraint);
+			var s = base.MeasureOverride (constraint);
 
-			var s = new System.Windows.Size (0, 0);
 			if (ScrollViewer.GetHorizontalScrollBarVisibility (this) != ScrollBarVisibility.Hidden)
 				s.Width = 0;
 			if (ScrollViewer.GetVerticalScrollBarVisibility (this) != ScrollBarVisibility.Hidden)