SelectionMouseHandler: Fix crash when TextArea.Document is re-assigned to a different document instance while the mouse button is down.

icsharpcode/ILSpy#1862

Author: dgrunwald

ICSharpCode.AvalonEdit/Editing/SelectionMouseHandler.cs

@@ -76,6 +76,7 @@ void ITextAreaInputHandler.Attach()
 			textArea.MouseMove += textArea_MouseMove;
 			textArea.MouseLeftButtonUp += textArea_MouseLeftButtonUp;
 			textArea.QueryCursor += textArea_QueryCursor;
+			textArea.DocumentChanged += textArea_DocumentChanged;
 			textArea.OptionChanged += textArea_OptionChanged;
 
 			enableTextDragDrop = textArea.Options.EnableTextDragDrop;
@@ -91,6 +92,7 @@ void ITextAreaInputHandler.Detach()
 			textArea.MouseMove -= textArea_MouseMove;
 			textArea.MouseLeftButtonUp -= textArea_MouseLeftButtonUp;
 			textArea.QueryCursor -= textArea_QueryCursor;
+			textArea.DocumentChanged -= textArea_DocumentChanged;
 			textArea.OptionChanged -= textArea_OptionChanged;
 			if (enableTextDragDrop) {
 				DetachDragDrop();
@@ -132,6 +134,15 @@ void textArea_OptionChanged(object sender, PropertyChangedEventArgs e)
 					DetachDragDrop();
 			}
 		}
+
+		void textArea_DocumentChanged(object sender, EventArgs e)
+		{
+			if (mode != MouseSelectionMode.None) {
+				mode = MouseSelectionMode.None;
+				textArea.ReleaseMouseCapture();
+			}
+			startWord = null;
+		}
 		#endregion
 
 		#region Dropping text
@@ -379,6 +390,10 @@ void textArea_QueryCursor(object sender, QueryCursorEventArgs e)
 		void textArea_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
 		{
 			mode = MouseSelectionMode.None;
+			if (textArea.Document == null) {
+				// Avoid entering any selection mode when there's no document attached.
+				return;
+			}
 			if (!e.Handled && e.ChangedButton == MouseButton.Left) {
 				ModifierKeys modifiers = Keyboard.Modifiers;
 				bool shift = (modifiers & ModifierKeys.Shift) == ModifierKeys.Shift;