Created attachment 582808 [details] Test case User Agent: Mozilla/5.0 (Windows NT 6.1; rv:8.0) Gecko/20100101 Firefox/8.0 Build ID: 20111104165243 Steps to reproduce: On frequent change of the progress element value it is redrawn and the throbber (? not sure if it's good name for it - I mean the slight pattern going from the left to right side in Win7) starts going from the beggining. The best will be to look at the test case and compare it in Fx8 and Ch16
I can't test this on Windows right now. It might be interesting to try on Windows 7 and Windows XP. Also, what about MacOS? On GTK things are working as expected. By the way, this is likely a widget bug instead of a DOM bug but I will wait until we know what is happening before changing the component. (and this is not a follow-up from HTML5 Forms but from a progress element related bug)
On Mac, using a Nightly, the testcase works well.
This bug is indeed reproducible on Windows 7 using Firefox 8. Very likely using a Nightly too.
I think what is wrong here is that the chunk position is computed using the bar size so if the bar size increases, the chunk position moves (goes a bit behind actually) which gives this weird behavior.