Investigate timeline scrolling issues

[turt2live]

There are a number of issues around scrolling in the timeline that, individually and together, can make for a quite poor experience in certain cases:

• Scrolling: Stuck in scroll loop #23208
• Scrolling: Clicking on a permalink often ends up with the permalink being scrolled off the screen #23393
• Scrolling: "Jump to first unread message" / Read Marker is really annoying element-meta#1418
• Scrolling: Improving jumps on timeline pagination/scrolling #8565
• Scrolling: Ability to scrollback can get stuck #18325

Before engaging in a larger rewrite of the timeline code, we'll need data points on whether or not it's possible to fix these issues with reasonable time investments. Therefore, the idea is to attempt fixes on two of the existing issues under this epic and use the outcome to drive further decision making.

• Fix scroll-jumps when jumping to a permalink matrix-org/matrix-react-sdk#9340 / Scrolling: Clicking on a permalink often ends up with the permalink being scrolled off the screen #23393
• Scrolling: Stuck in scroll loop #23208

Time sheeting

Project: Web: Scrolling

[daniellekirkwood]

Also look into the Jump-to-date feature

[claell]

@Johennes Can you give a short update on the course change you mentioned in #23540 (comment)? Seems like before it was planned to fix the issues, and now it's an investigation first. What happens if it turns out the issues can't be fixed with reasonable time effort? Leave them as they are?

[Johennes]

#23540 meant to prepare a full rewrite of the timeline to fix the various issues in this epic. After reconsidering, we concluded that this isn't a good next step. For one thing, replacing the timeline code requires a mighty investment of effort. For another, most of the issues listed here didn't exist at some point which hints at the possibility to fix the current implementation without a rewrite. The course change is to investigate that latter option. We'll reconsider the subsequent once this is completed – whether successfully or not.

[claell]

Sounds good. Though also the description of this issue has been changed from fixing to investigating (and estimating the effort). But in general seems to make sense. As a user, I just hope, this can see some priority, so that matrix-org/matrix-react-sdk#9340 can get merged which might also fix some other stuff as mentioned in matrix-org/matrix-react-sdk#9340 (comment).
As always, I am available for testing and reproducing, also via video call for example.

[Johennes]

Sounds good. Though also the description of this issue has been changed from fixing to investigating

Yes, we split the two so that we can schedule them separately in the roadmap. So the idea would be to have another epic for the actions that come out of this one.

I just hope, this can see some priority, so that matrix-org/matrix-react-sdk#9340 can get merged

Yes, matrix-org/matrix-react-sdk#9340 claims to fix #23393 which is one of the two tasks under investigation in this epic. So I would expect us to revisit that PR as part of this project. I've added it into the checkbox in the issue description to make this more clear.

[andybalaam]

Comment from @MadLittleMods about potentially-related issues. Dropping here in case we want to include them when we work on this:

In addition to timeline scroll problems, there is a timeline reliability problem:

• Missing thread replies: Thread reply missing from thread in the mix of sending my own thread reply #24466
• Missing edits: Element not showing message edit #24515
• Most of the timeline is missing: Most of the timeline is missing #21432
• Timeline is completely out of order: After jump to unread, timeline is out of order and showing a thread message in the main timeline #21922
• Timeline is duplicated/looping/repeating:
  • The conversation in the room is duplicated 8 times #25195
  • Looping/Repeating messages and missing messages #24354
  • Thread timeline repeats #24676
  • Duplicate events in the timeline #12471
• Jumping to unread doesn't work half of the time:
  • Clicking on the jump to unreads button throws a Failed to load timeline position error and dumps me in a blank timeline #20927
  • Jumping to oldest unread message doesn't load timeline properly #22989
  • Timeline fails to load after clicking the ^ to go to the first unread message #20397