I have a problem with Firefox Beta 52. It also happens in Beta 53, Nightly 55. Doesn't happen in ESR 45. On some sites I noticed a very weird behavior: mouse movements are captured incorrectly, as if mouse cursor was placed in a different area. I experimented a bit, and created a working example of the bug. I'll check those sites once this bug is fixed. Here's how to reproduce the bug: 1. Open attached page 2. Click on the button, then immediately move mouse over iframe 3. Keep moving mouse up and down within iframe and right-clicking, but don't click on buttons. Result: buttons become hovered far away from mouse cursor; context menu appears far away from mouse cursor. Expected: normal behavior. I've added "[e10s]" in the summary, because it isn't reproducible if in Nightlly I disable "multiprocess". Please correct me if this logic is incorrect.

Another bug detected with that page is bug 1351063.

I found a regression range in FF42: https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=b0b3dcfa5557&tochange=d3228c82badd I think it's a side-effect of bug 1172239 with e10s enabled.

I'm hoping this is more of a Layout thing :)

Too late for a fix for 53, fix-optional for 54, minor carryover regression.

Astley, should this be fix-optional for 57?

Extracted iframe content to a separate HTML file.

Refresh the test case as iframe with src attribute in data URI format will be taken as unique, opaque origins. See bug 1324406.