If a div with a border-radius has another element inside of it, and that element is resized, the redraw of the div with the border-radius will be messed up. This bug seems to occur when the resize is called from (at least) an onClick() or a setTimeout(). However, if the resize is called in the body onLoad, or if it's enclosed in a script tag executed on the page's loading, the div will redraw properly.
Created attachment 10374 [details] reduction/test case
I can reproduce this with ToT. It appears to be an invalidation issue. Thanks for the great reduction Mike!
*** Bug 11098 has been marked as a duplicate of this bug. ***
Bug 6301 was similar to this, and I think you can improve the fix for that bug to cover this bug as well.
Created attachment 12045 [details] Account for border-radius on the moving edges
Comment on attachment 12045 [details] Account for border-radius on the moving edges r=me
Landed in r18452.