Detect mouse capture loss during text selection

There may be times when SelectionMouseHandler detects a left mouse button down event on its TextArea, but then immediately the mouse capture is lost, and SelectionMouseHandler does not reset the selection mode. 

I noticed this when I started showing a dialog in response to my main window being activated, if I have detected that the file being edited by the user was changed by another process. When I activate my application by clicking on the editor, the SelectionMouseHandler detects this and begins selecting text. But immediately the TextArea loses mouse capture, as in my handler I show a modal dialog. When the user dismisses this dialog, event though the left mouse button is up, moving the mouse around extends the selection as if it were down.

I mimic exactly the approach in the source code for System.Windows.Controls.Primitives.Thumb, the only difference being that here I have to go looking for the SelectionMouseHandler that is associated with the TextArea in the sender parameter. Perhaps there is a more direct way of finding it, though this seems to work for me.

Author: jimfoye

ICSharpCode.AvalonEdit/Editing/SelectionMouseHandler.cs

@@ -89,6 +89,22 @@ public SelectionMouseHandler(TextArea textArea)
 			this.textArea = textArea;
 		}
 
+        static SelectionMouseHandler()
+        {
+            EventManager.RegisterClassHandler(typeof(TextArea), Mouse.LostMouseCaptureEvent, new MouseEventHandler(OnLostMouseCapture));
+        }
+
+        private static void OnLostMouseCapture(object sender, MouseEventArgs e)
+        {
+            TextArea textArea = (TextArea)sender;
+            if (Mouse.Captured != textArea)
+            {
+                SelectionMouseHandler handler = textArea.DefaultInputHandler.NestedInputHandlers.OfType<SelectionMouseHandler>().FirstOrDefault();
+                if (handler != null)
+                    handler.mode = SelectionMode.None;
+            }
+        }
+
 		public TextArea TextArea {
 			get { return textArea; }
 		}