This behavior is caused by the RadRichTextBox control handling the mouse down event and not handling the mouse up event, which respectively stops the hit testing and deselects the shape. We are currently working on resolving this. In the meantime, what you can try is to handle the PreviewEndEdit event of the RadDiagramShape class and handle it so that the shape does not exit edit mode. This needs to be done only when the rich textbox control has been hit (mouse left down).
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