User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.13; rv:60.0) Gecko/20100101 Firefox/60.0 Build ID: 20180203100135 Steps to reproduce: Open a new tab. Actual results: Activity Stream preferences are overlayed with the actual content, on the left. The are not interactable. If i hit the cog icon in the upper right, the preferences move to the correct location on the right and can be interacted with. Closing them moves them back to the left Expected results: No preferences visible.

This is probably WONTFIX given bug 1432672.

Do you have any custom styling, e.g., userContent.css? As Gingerbread Man mentioned, the preference sidebar is going away with bug 1432672 for Firefox 60.

No, short of enabling webrender Nightly is completely stock. Disabling webrender doesn't make the issue go away.

If you want to dig in a bit more, from the broken page, what's the html hierarchy to `<div class="sidebar` ? And if you play around with the css rule for `.sidebar.hidden`, does that affect the visibility? In particular `.sidebar` should have a `offset-inline-end: 0` that positions it on the right side of the screen, but your screenshot seems to indicate multiple styles not applying (while some are)…

Some peculiar behavior (see attachments). Opening dev tools partially resolves issue. Selecting the item in the inspector hides the broken sidebar entirely. Closing devtools makes it reappear. I've captured the outer HTML of the sidebar element, but I used the devtools to do so, so at that point the "busted" side bar was gone. I initially thought the visibility change might be related to the size of the viewport, but changing the window size doesn't alter the presentation of the sidebar, just opening and closing the dev tools

Added source from "view selection source" of the whole page. Note that I can't actually highlight the side bar directly (as stated in the original bug, I cannot interact with the broken sidebar) but the text does highlight when I use Meta+a

I have the same issue, and disabling "offset-inline-end: 0" removes it. In another profile, I can only reproduce it after opening the devtools at the bottom, but closing or moving the devtools won't change. It returns to normal after a refresh when the devtools is not at the bottom. It only happens to me with WebRender enabled. I'm on Arch Linux.

Upon further investigation I am an idiot, and indeed, on my Mac, disabling webrender DOES remove the issue. I can also verify that at least the outer HTML of the sidebar is the same in both webrender enabled and disabled modes.

bug 1436420 has a testcase