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).
