Reader: scrolling problems in Reader

[kristarella]

Steps to reproduce

1. Starting at URL: https://wordpress.com/tag/photography (I think it happens on any page, but this tag makes it easier to see)
2. Scroll, keep scrolling
3. Observe the scroll marker on the right of the browser
4. Observe images

What I expected

To see smooth-ish scrolling on the right.
To see each post once.

What happened instead

The scroll marker jumps rather eratically.

Posts appear to be duplicated, but I'm pretty sure it is just that the page is jumping back to a previous point.

Observe in the video below that when I click a link, and then see the same image again, the link is still outlined, which is why I think it is page jumping, not posts actually being duplicated.

Browser / OS version

Firefox 70.0.1
Mac OS X Mojave
Windows 10
Maybe Edge also?

Screenshot / Video

https://cld.wthms.co/tn6FmL

Context / Source

#user-report
2505176-zen

[bluefuton]

Thanks for writing this one up @kristarella! I've also seen this since switching to Firefox recently. No solution yet but it's on my radar.

[Agepeuve]

Another report in the ES forums - https://es.forums.wordpress.com/topic/problemas-con-el-deslizamiento/

[lakellie]

Another report in 2520151-zen and the user mentioned they are experiencing the issue in both Firefox and Safari

[pento]

I've run through a git bisect, it seems like this was caused by #37438.

From the debugging I've done so far, it appears that this call returns null, because boundsForRef() isn't finding the ref.

@bluefuton, @jsnajdr: I've run into the limits of my knowledge of how this magic works, and my random hacking is not producing anything of value. Could you review this, and help with the next steps?

[blowery]

@pento taking a look. I bet we need to forward a ref somewhere.

[jsnajdr]

Fixed by #38335, regression introduced in #37553 and #37438. Thanks @kristarella for reporting and @pento for bisecting and debugging! Everyone is invited to test and review the fix.

[bluefuton]

High fives all round @kristarella, @pento, @blowery and @jsnajdr. Thanks for getting this fixed ❤️

[pento]

Thank you for fixing this, @jsnajdr! 💖