Splitter leaving trail in winforms datagridview custom editing control

Question

I've created a custom column for the datagridview. The editing control consists of a user control with a multline textbox and button placed on it.

This all works fine unless I size a column left or a row up. The part of the splitter over the background of the user control is fine, but the part over the textbox or button leaves a trail behind it.

I've worked around this by adding a handler to the RowPostPaint event, when initialising the editing control. It's certainly an improvement and removes the trails, but now the splitter flickers on the portion that is over my editing control.

I've used reflector to see if there's special handling in the microsoft editing controls but I can't see anything. I suspect it has something to do with a user control.

Any ideas?