Stop mouseDown event from propagating.

Closes #2459

src/editor-component.coffee

@@ -462,6 +462,8 @@ EditorComponent = React.createClass
         when 3 then editor.selectLine()
 
     @selectToMousePositionUntilMouseUp(event)
+    e.preventDefault();
+    e.stopPropagation();
 
   onStylesheetsChanged: (stylesheet) ->
     @refreshScrollbars() if @containsScrollbarSelector(stylesheet)