On second glance, the way that we determined a media tile to be 'waiting for media' was too implicit for my taste. It would appear on a surface reading to depend on whether a participant was currently publishing any video. But in reality, the 'video' object was always defined as long as a LiveKit participant existed, so in reality it depended on just the participant. We should show this relationship more explicitly by moving the computation into the view model, where it can depend on the participant directly.
We would show 'waiting for media' on participants that were connected but had no published tracks, because we were filtering them out of the remote participants list on connections. I believe this was done in an attempt to limit our view to only the participants that have a matching MatrixRTC membership. But that's fully redundant to the "Matrix-LiveKit members" module, which actually has the right information to do this (the MatrixRTC memberships).
This fixes a regression on the development branch: the layout switcher would not respond to input while the window mode is 'flat' (i.e. while a mobile phone is in landscape orientation). See https://github.com/element-hq/element-call/pull/3605#discussion_r2586226422 for more context.
I was having a little trouble interpreting the emergent behavior of the layout switching code, so I refactored it in the process into a form that I think is a more direct description of the behavior we want (while not making it as terse as my original implementation).
Semantically, behaviors are only meaningful for as long as their scope is running. Setting a behavior's value to an empty array once its scope ends is not guaranteed to work (as it depends on execution order of how the scope is ended), and subscribers should be robust enough to handle clean-up of all connections at the end of the scope either way.
