Set e.Handled=true in TextArea.Drop handler.

src/Libraries/AvalonEdit/ICSharpCode.AvalonEdit/Editing/SelectionMouseHandler.cs

@@ -206,6 +206,7 @@ namespace ICSharpCode.AvalonEdit.Editing
 								textArea.Document.UndoStack.EndUndoGroup();
 							}
 						}
+						e.Handled = true;
 					}
 				}
 			} catch (Exception ex) {