I have a mammoth GitHub PR that seems to routinely generate issues.

STR

1. Open https://github.com/quicwg/base-drafts/pull/4059
2. Scroll up and down. Best (or worst) results are achieved by scrolling to the bottom and then scrolling up; you might have to scroll to the bottom and then reload to get this.

After a while the page begins to stop rendering and I get solid white. I get the little banner at the top, but nothing else. (That banner flashes in and out as I scroll, but it remains.) After scrolling up a while, the main content of the page stops rendering.

I've taken two screenshots using two tools, which shows a possible clue. The first, taken with Windows (Alt-PrtSc) shows the rendering issue. The second, taken with the Firefox screenshot tool shows a broken image when previewing it, but when pasted into another app shows the very bottom of the page, as though the scrolling never occurred.

I am using webrender (gfx.webrender.all = true) and I also found:

general.smoothScroll.mouseWheel.migrationPercent = 0
general.smoothScroll.msdPhysics.enabled = true

All the apz. prefs are at their defaults.

I might have neglected to mention that this is Nightly 82.0a1 (2020-09-13) (64-bit). The same problem was present in the previous build as well.

This is likely the same problem as bug 1664838, I'll take a look.

Based on my investigation of bug 1664838 I'm 95% sure this bug has the same underlying cause, and should be fixed by the patch on that bug. However I wasn't able to reproduce this bug using the given STR, so I can't confirm that. I suspect the STR are missing some required steps, probably including loading the page initially in a background tab, or something along those lines.

If you're able to reproduce this on a fresh browser start, feel free to try the build from the try push linked here, or wait until tomorrow's nightly, which I expect to have the fix.

I'll leave this open for now but intend to dupe it over to bug 1664838 if it's not reproducible with the patch on that bug.

Thanks Kats, I can confirm that I missed the "hit reload and then switch to another tab so that it loads in the background". That works reliably for reproduction with my current build. The page loads, renders, and scrolls perfectly in the try build you reference. I'd say you can improve your confidence to 99% and resolve this one.

Great, thanks for checking!