This is more general maintenance than performance as the RoomList doesn't need to be generating layouts for the sublists, and it certainly doesn't need to be creating a bunch of extra ones.
The sublists are perfectly capable of getting their own layout instance and using it, and we are perfectly able to limit the number of these things we create through the session's lifespan.
In this demonstration, we remove the cutting line (as it collides with the tile in a weird spot) and instead replace the tile with a placeholder when the text is about to collide with the avatar in the tile. We use a `round()` for this because through some amazing coincidence the collision happens at 0.47, which is close enough to 0.5 for people not to notice.
The layout updates are anecdotal based on devtools flagging the values which are "changing" even if they aren't. The scrolling feels better with this as well, though this might be placebo.
We now use offsets and scroll information to determine where the headers should be stuck to, still supporting the transparent background.
Some scroll jumps were originally introduced as part of the change in numbering, so they have been fixed here. By proxy, some additional scroll jump/instability should be fixed as well.
This has a lingering problem of still causing a huge number of no-op UI updates though, which will be dealt with in a future commit.
It should be in all major browsers as of years ago, and we use it unguarded elsewhere in the app. The performance benefits of it appear to be worthwhile enough to keep it, though it's not a perfect solution.
Fixes https://github.com/vector-im/riot-web/issues/14378
Rooms transitioning between multiple states are often at risk of going missing due to the sticky room handling. We now protect that transition by bluntly ensuring the room can't go missing, and by always ensuring we have an updated reference to the room.
react-resizer appears to be okay at tracking state, but it often desyncs from reality. re-resizer is more maintained and more broadly used (160k downloads vs 110k), and appears to generally do a better job of tracking the cursor.
The new library has some oddities though, such as deltas, touch support (hence the polyfill), and calling handles "Enable".
For https://github.com/vector-im/riot-web/issues/14022
