I plan to discuss style resolution in relation to CSS transitions with David Baron soon, but Gavin wanted me to file a bug and reference it in the patch I'm about to land, so here it is. A typical block of code that I write to skip the animation once is: CSS: #foo { width: 50px; -moz-transition: width 1s; } JS: var foo = document.getElementById('foo'); foo.style.MozTransition = 'none'; foo.style.width = '100px'; foo.clientTop; // force style resolution foo.style.MozTransition = ''; Using element.clientTop in this way is rather common in our codebase: https://mxr.mozilla.org/mozilla-central/search?string=.clientTop%3B Gavin considers this to be a hack and would like some investigation and dicussion on why we need this or how to make it better. Last week, I made a few testcases for this here: http://frankyan.com/labs/racy.html It is so named for the race conditions. Whether or not the transitions occur seem to be unpredictable for the last two boxes on Linux.

So, a few thoughts here: * it would probably be good if we had common functions for flush_style, flush_frames(), flush_layout(), etc. In this case we only need flush_style() (which you can do with something like getComputedStyle(document.documentElement, "").color -- at least so that we can spot intentional flushes easily, and perhaps work on figuring out how we can avoid them * a possible solution here would be an API like the one I proposed here: http://lists.w3.org/Archives/Public/www-style/2011Mar/0729.html though the only response so far: http://lists.w3.org/Archives/Public/www-style/2011Mar/0730.html is not exactly supportive. With an API like this, if you're doing *all* your transitions from script, you could just use this API rather than using style-change-triggered transitions.

> * it would probably be good if we had common functions for flush_style, > flush_frames(), flush_layout(), etc. Yes! Can we put them somewhere where reftest can trigger them too?

Frank: What is the concern that caused you to mark this a security bug?

Accidental. Feel free to unmark.