User Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:34.0) Gecko/20100101 Firefox/34.0 (Beta/Release) Build ID: 20140826030203 Steps to reproduce: STR: Create new profile with Firefox Nightly, 20140826030203. Go to any webpage that is too wide to be complete seen and thus has a horizontal scrollbar. (One can be created by zooming in, if necessary, on any page.) Scroll vertically down part way through the page. Click on the horizontal scrollbar. Actual results: Actual results: Firefox scrolls back up to the top of the page. Expected results: Expected Results: Horizontal scrollbar should result in horizontal scrolling.

I can reproduce this on Windows 8.

m-c regression window: https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=daa84204a11a&tochange=dc352a7bf234

This tentatively looks like bug 957445...

Confirmed by checking against inbound. Mats, can you take a look?

And , Dragging thumb of vertical scrollbar also trigger a reset of the horizontal scroll position.

I can confirm that bug 957445 caused this regression, using a local build. Part 2 to be precise.

I have reproduced on OSX 10.9 and am investigating the cause.

kip, I have debugged this and this appears to fix it: static nsRect GetOnePixelRangeAroundPoint(nsPoint aPoint, bool aIsHorizontal) { - nsRect allowedRange; + nsRect allowedRange(aPoint, nsSize()); nscoord halfPixel = nsPresContext::CSSPixelsToAppUnits(0.5f); I can do some more testing locally and then let you review it? (feel free to test it on OSX, I'm on Linux)

Reproduced with Nightly 2014-08-27 on Windows 7 x64. Verified as fixed with Firefox 34 beta 7 (Build ID: 20141106201515) on Mac OS X 10.9.5, Windows 7 x64 and Linux 14.04 x32.

Moving from Core::Untriaged to Core::General https://bugzilla.mozilla.org/show_bug.cgi?id=1407598