I don't see any easy way around this. My guess is you would need to use a CreationFilter either instead of or in addition to your DrawFilter. The CreationFilter allows you to change the size and position of elements.
Can you post a screen shot of what this looks like? My guess is that your DrawFilter will also have to draw the focus rectangle. The tree can't possibly know how big the text is when you are drawing it.
I'm not sure if this is possible, and if it is possible, how to accomplish it. Further research will be needed.
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