An inline block with clipped overflow makes the line's vertical overflow increase to accommodate its overflow as if it isn't clipped. This becomes evident when the line is inside a table cell, since cells grow to include overflow. I'm going to attach a test case and a fix soon.
Created attachment 7433 [details] Test case
Created attachment 7435 [details] Patch including layout test and change log
Comment on attachment 7435 [details] Patch including layout test and change log r=me
Verified in r13675 nightly.