The described behavior in your post is the default one. To start the TextTool while the mouse is over a shape can be achieved using custom code. I have created a sample project to demonstrate one way of achieving this. In a few words, I have subscribed to the PreviewMouseDown event of the RadDiagramShape and handle the event. Then the same mouse down event is raised on the RadIDagram itself which triggers the TextTool creation logic. As a note, I am setting the IsEnabled property of this shape to false and enabled it again in PreviewMouseUp events. Check the attached project.
Hope this solution will work on your side.
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