I love the idea of a helper function that is able to toggle a little indicator next to the extension icon, as it is minimally invasive (which IMO for things like notifications is essential, anything that takes away from the viewing experience is a detriment to the overall stream and having a simple coloured dot next to the extension is as minimal as you can get) and extension devs could easily have a setting to disable triggering it should the user not wish to have notifications.
Actual notification messages I’m not as sure about. There’s certainly a use for it but then there’s the question of is a single area of the screen, with static styling, suitable for all channels/extensions? There’s also the issue of what should be the default? I think the default viewer experience should be ‘Just the stream’, and that things like notifications should be opt-in. This could be tied to identity sharing so agreeing to that enables the notifications for that extension (with the option to still hide them if they wish) but even then there may be extensions that don’t require identity sharing but do want notifications.
Yes, we are lifting that limit on the width of components. However note a 16:9 aspect ratio using 100% target height is not going to consume the entire player, as “100%” there is going to mean the full distance between the top and bottom player controls.