This is a continuation of bug 146831, which I'm closing after the first and main patch has been checked in. That patch improves performance significantly. However, there are still significant performance problems (although not full CPU usage) on Windows Mozilla (but not mfcembed) and on Chimera/ProjectX. The approach in the "reflow roots hack" on bug 146831 should be investigated. We should also investigate removing the extra reflow we do for incremental reflows within absolutely positioned elements to determine the overflow area.
Isn't this somehow related to bug 75121 ?