1. Open a simple QML file in Designer:

   import QtQuick 2.3
   import QtQuick.Window 2.2
   
   Window {
       visible: true
       width: 640
       height: 480
   
       Text {
           id: text
           anchors.centerIn: parent
           text: "This Text is long"
       }
   }
   

2. Double-click the text item.
   The text is being selected and could be changed now.
3. Move the mouse pointer to the right of the Text item.
   You'll see something like this:
   

Designer should only show the selected text until the user finished editing. The it should show the QML rendering again.