Added CaptureMouse() for drag fix #306

Mouse needs to be captured at the start, to listen mouse move events before document IsActive is set.

source/Components/AvalonDock/Controls/LayoutDocumentTabItem.cs

@@ -109,6 +109,7 @@ protected virtual void OnModelChanged(DependencyPropertyChangedEventArgs e)
 		protected override void OnMouseLeftButtonDown(MouseButtonEventArgs e)
 		{
 			base.OnMouseLeftButtonDown(e);
+			CaptureMouse();
 			_allowDrag = false;
 			Model.IsActive = true;
 			if (Model is LayoutDocument layoutDocument && !layoutDocument.CanMove) return;