Sometimes there are bugs where we least expect them, even in the basic controls. In your case are using a Canvas
panel in the TemplateFieldsAdministrator
which is causing the issue.
The canvas panel sometimes behaves oddly and messes up the layout of seemingly unrelated controls. Also if there is no Background set, sometimes it will decide to "handle" mouse events so controls in or around it will behave strangely.
In your case:
1. Just comment out the Canvas panel,
2. Call InvalidateMeasure and UpdateLayout for the Canvas or like so:
Generally you will save yourself some headaches if you avoid the Canvas panel :)
the Telerik team