Confirmed with Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:1.9.3a4pre) Gecko/20100318 Minefield/3.7a4pre This is a regression from Firefox 3.5.
Regression range: works - "Show Overlay" retains position: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.2a1pre) Gecko/20090511 Minefield/3.6a1pre http://hg.mozilla.org/mozilla-central/rev/ff250122fa99 broken - "Show Overlay" scrolls to top: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.2a1pre) Gecko/20090512 Minefield/3.6a1pre http://hg.mozilla.org/mozilla-central/rev/ed38105c9c2a http://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=ff250122fa99&tochange=ed38105c9c2a
maybe an outcome of Bug 78070?
Changing the "overflow" property reframes the element. For the root, this was indeed changed in bug 78070; for other things it's always been the case. I believe we have existing bugs on not persisting scroll position across reframe.
Attached testcase works fine.