I'm seeing this weird behavior maybe a few times a day where if I use horizontal scrolling in the tab bar, I get the history back/forward swipe animations as if the event was targeted at the content area. Moving the mouse pointer over to the content area and then back to the tab bar fixes this. Note that I use an Apple external mouse and the horizontal scrolling and swiping gestures are the same (one finger scrolling to left or right.)

This makes it next to impossible to scroll any document horizontally. I get bitten by this on a daily basis on websites such as trello where the UI is built around horizontal scrolling...

Ehsan, can you reproduce with current nightly?

(In reply to Brad Lassey [:blassey] (use needinfo?) from comment #2) > Ehsan, can you reproduce with current nightly? Yes.

NI to George to sit next to Ehsan and figure out what's going on here

FWIW, which may not be much, this is the expected behavior when the cursor is over the addressbar/searchbar/toolbar icons (just not the tabs). The behavior holds for the entire light-grey region, which extends a little higher than you might expect -- it seems to extend about 2-3 pixels above the shadow between regions. Of course, this is also the behavior in non-e10s.

Ehsan's comment 3 refers to a different bug, and he says he can't reproduce this anymore. Let's close this, and Ehsan is going to try and hunt down the other bug or file a new one for the issue he's currently seeing (it's likely a dupe of bug 863514)