rationally in bug 714579 comment 10:
getComputedStyle on an element in document D can lead to destruction of any presshell in the document tree containing document D.
that means getComputedStyle can destroy whole document accessible and if we are calling this method internally then we get into trouble. We can't check all code paths where getComputedStyle is used and add IsDefunct checks so we need to switch to non-flush version (general bug 728126). Now we're going to borrow layout code to make this (approach from bug 714579).