User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.13; rv:59.0) Gecko/20100101 Firefox/59.0 Build ID: 20180315233128 Steps to reproduce: I gave an element the falling styles. position: sticky top: 0 float: right Actual results: The page renders as expected, but if I scroll down the page and reload, floated elements further down the page are moved out of the way of the sticky element at it's new position. Sometimes they will jump back to the correct position. Couldn't recreate the same result on a sample project. It might have to do with the complexity of the page I am working on. I could create a sightly different issue. https://piquant-apple.glitch.me/ Renders fine. When I open the inspector, the black boxes will jump down the page to be below the sticky element. Seems like opening and closing the inspector will cause the blocks to reposition themselves. Expected results: The rendered result should be the same.

I can reproduce the issue in Nightly61.0a1.

Regression window(White space displayed at the bottom of the page): https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=2ae8554e0989e12e9ef5755a4171841b02df7f11&tochange=07de05d8060cb97df8df4aaf371b7f23da4b46cb Regressed by: 07de05d8060c Matteo Ferretti — Bug 1312103 - Avoid scrolling latency on highlighters given by APZ; r=pbro

Note that, as shown in the video (attachment 8962559 [details]), this only occurs when the highlighter is ON while scrolling. So you first have to go into the "element picker" mode, hover over the page, and then scroll. If you don't have the highlighter ON while scrolling, the problem does not occur. So it's a case of the highlighter's markup or style forcing a different layout on the page. This is a pretty bad problem, the tools should never impact the layout of the page other than by an intentional user action.