A WebCompat bug was originally filed here: https://webcompat.com/issues/2601 Copying the STR: 1) Navigate to: https://www.codecademy.com/en/courses/learn-java/lessons/introduction-to-java/exercises/whats-your-name 2) Once the site loads, try to adjust the mini-window(s) positions by moving it left or right using the dividers that separate them. The problem was reproduced by hallvors on a Linux x86 machine, running Firefox 49.0, in single-process mode. A performance profile was gathered here: https://cleopatra.io/#report=ded0eb663276b8ee1788fdadd8cc887829a78c32&invertCallback=true&selection=%22(total)%22,706,281 It looks like we're spending a large amount of time doing reflow. About 5 total seconds from the profile can be attributed to reflow. It's possible that this page just needs to not cause so much reflow (hence the WebCompat bug, and jrmuizel's comment at https://webcompat.com/issues/2601#issuecomment-224593384), but I'm filing this here anyways in case there's anything we can do in Gecko to alleviate this kind of situation.
I would add another thing to this issue on this specific site here. Set the Firefox window into non-maximized mode Drag the handles to resize the Firefox window. Notice black or white area's and the resizing feels janky. On another browser even though there is a delay its not this slow/janky. On certain other links of the same site the resizing feel more bad in e10s than non-e10s mode(dont know why)
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