Buffer expansion offset issue

Hi. I'm modifying a textbox plugin and using smartfx. To simplify things I make the output the size of the entire buffer in preRender:

UnionLRect(&req.rect, &extra->output->result_rect);

UnionLRect(&req.rect, &extra->output->max_result_rect);

This works well, the problem comes if I apply a buffer-expanding effect afterwards such as blur or drop shadow. This offsets the textbox effect for some reason.

Correct:

Incorrect as a blur has been applied after the textbox, and has offset it for some reason:

Any ideas for a different method to have the output the size of the comp without this offset issue? Thanks.