Scrolling bounces back when page changes size

I can't figure out if I can edit the title and if so, how to do it, but I've had it happen a couple of times even when the page doesn't change size, so maybe it's unrelated to that. I just had more luck reproducing it consistently when it does. Once the page is all loaded it doesn't happen as much. It's possible some pages load / unload elements both when scrolling down and when scrolling back up though.

For example it happens a lot on GitHub PR pages.

Ok this is trickier than I thought, it does happen is Safari as well, for example on this page: https://daringfireball.net/2021/10/the_tragedy_of_safari_15_quote_unquote_tabs. It only happens while images are displayed on screen, as soon as the images are scrolled out of view it stops happening. It happens very consistently. I can't see the page changing size so it's probably unrelated to that.

Checked both Firefox and Chrome again, neither of them have this issue, so maybe a WebKit issue?

Perfect, cannot repro on 264677@main ! How quickly can we pick up new versions of WebKit?

I'm not familiar enough with WebKit to be able to tell which commit fixed this, there's a lot of changes to scrolling code in the recent commits.

Repro steps are:

1. Open https://daringfireball.net/2021/10/the_tragedy_of_safari_15_quote_unquote_tabs
2. Swiping with two fingers on the trackpad, scroll down the page, about 5 lines at the time, lifting your fingers trying not to flick.

If it repros you'll notice the page scrolls back in the opposite direction 1-2 lines every time you lift your fingers, but only if there are images being displayed. Once you scroll past the images and there's only text it doesn't happen anymore. The page scrolls back up if you were scrolling down, and scrolls back down if you were scrolling up. If you flick it doesn't happen because it will continue scrolling in the same direction and then come to a stop.

It reproes very consistently so I could easily test it on all my browsers, including Safari using the new WebKit.

Yeah I just did, it still reproes.
To use the latest WebKit you go to https://webkit.org/build-archives/, download the WebKit build you want, run sudo xattr -r -d com.apple.quarantine on all the files (they're not signed), then run the run-webkit-archive script that comes in the zip file.
I also tried launching Orion with that script, but I could still repro the bug, so I guess it didn't quite work, maybe because Orion already has its own version of WebKit, so setting the DYDL_* environment variables isn't enough.

Would be cool to have a /Applications/Safari.app/Contents/MacOS/SafariForWebKitDevelopment equivalent for Orion.

Done: https://orionfeedback.org/d/5301-provide-an-equivalent-to-safariforwebkitdevelopment.