It doesn't make sense to preserve EventHandler::m_mousePressNode in a cached frame (and we don't preserve EventHandler::m_mousePressed anyway, potentially breaking EventHandler invariants).
How m_mousePressNode got into CachedFrame was by mechanically combining PageState and PageCache in r20837.
Created attachment 229294 [details]
Comment on attachment 229294 [details]
Clearing flags on attachment: 229294
Committed r167267: <http://trac.webkit.org/changeset/167267>
All reviewed patches have been landed. Closing bug.