User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:20.0) Gecko/20130130 Firefox/20.0 Build ID: 20130130042019 Steps to reproduce: Open http://www.sorced.com/2012/03/penance-driven-development.html Note: in order to reproduce the issue sometimes you need to open different tabs of the same page Actual results: The pop up element to subscribe RSS covers part of the vertical scroll bar Expected results: The element should stay under the scrollbar

I fail to reproduce in Aurora/Nightly with HWA on/off. Is your Issue reproducible in Safe-Mode? https://support.mozilla.com/en-US/kb/Safe+Mode

I'm able to reproduce and I have better STR: 1) Open a 1st tab (whatever the URL) 2) Open a 2nd tab and load http://www.sorced.com/2012/03/penance-driven-development.html 3) Switch immediately and quickly to the 1st tab 4) After the 2nd tab is fully loaded, switch to it NB: Repeat steps 2 to 4 if necessary, the central point is to not keep the focus on the tab when the webpage is loading. Result: The black widget on the right covers the right vertical scrollbar: http://i.imgur.com/95BQ9cr.jpg If I keep the focus during the 2nd tab is loading, rendering is normal: http://i.imgur.com/d25uKfs.jpg It's an old regression from firefox-4.2a1pre: m-c good=2011-03-25 bad=2011-03-26 http://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=e11c2f95f781&tochange=bf68a4a3ef6a

Regression window(m-c) Good: http://hg.mozilla.org/mozilla-central/rev/8442a1f9aaba Mozilla/5.0 (Windows NT 6.1; WOW64; rv:2.2a1pre) Gecko/20110325 Firefox/4.2a1pre ID:20110325162049 Bad: http://hg.mozilla.org/mozilla-central/rev/256d72a8245c Mozilla/5.0 (Windows NT 6.1; WOW64; rv:2.2a1pre) Gecko/20110325 Firefox/4.2a1pre ID:20110325180928 Pushlog: http://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=8442a1f9aaba&tochange=256d72a8245c In local build: Last Good: 8442a1f9aaba First Bad: e328ab1bcb03 Triggered by: e328ab1bcb03 Boris Zbarsky — Bug 633421. Throttle timeouts in background tabs to 500ms, noot 10ms. r=jst WORKAROUND: It works if I set dom.min_background_timeout_value = 10

Odd. I mean, that change could have changed timing of thing, but shouldn't allow content to cover the scrollbar!

Based on the screenshots in comment 2 it looks like the content covers scrollbars from a scrollable element that is not the root scroll bar. I think that is expected. Where the scrollbar is changes between the 1st and 2nd screenshots, I don't think that is a coincidence. Why the page does that is the real question.

Oh, this isn't the viewport scrollbar? In that case there's a good chance this page is just doing some weird scripted layout computation and doing it at the wrong time now. Reduced testcase would help.

https://bugzilla.mozilla.org/show_bug.cgi?id=1472046 Move all DOM bugs that haven't been updated in more than 3 years and has no one currently assigned to P5. If you have questions, please contact :mdaly.