Display mode negotiation is missing

[antonpk1]

Currently the spec has:

• displayMode - host tells app the current mode
• availableDisplayModes - host tells app what modes it supports

But there's no way for the app to declare what display modes it supports.

For example, if an app wants to say "hey I support fullscreen", there's no mechanism for that. The host just assumes all apps work in all modes?

We need some negotiation where:

1. App declares its supported display modes (maybe in ui/initialize capabilities?)
2. Host knows its own supported modes
3. They intersect to determine what's actually available
4. App can request a mode change (currently also missing)

Right now the display mode stuff is one-directional and incomplete.

[martinalong]

4. App can request a mode change (currently also missing)

Instead of adding this, I think it might be simpler if hosts just control mode changes? Like if the "full screen" and "pip" buttons live on the host, and the host just informs the apps when the mode has changed. Since otherwise the app might request a mode at a time where it isn't possible and we'd have to handle "rejecting" that request, or might request full screen mode or pip without any user action which would be undesirable.

This makes it a simple two step handshake:

1. App tells host what modes (of "full screen" and "pip") it supports in ui/initialize
2. Host tells app which mode it is switching to (it factors in what modes it supports and what modes the app supports)

And way, we can also get rid of availableDisplayModes since the host doesn't need to inform the app of which modes it supports, since it's orchestrating which mode we are in

(Note that I excluded "carousel", because of the reasons outlined in #51)

[antonpk1]

@martinalong yep that makes sense to me 👍

[idosal]

@martinalong It makes sense and simplifies matters, but I can imagine cases where an App may benefit from changing its own display mode (e.g., expanding to provide more real estate so the user can complete their intent within the app). However, they might not be encouraged from a philosophical standpoint

[martinalong]

Agreed they might want to expand to provide more real estate, but I think the host should ultimately have the say in whether they're allowed to. Otherwise many apps could just full screen right away when they load in, or repeatedly trigger full screen, preventing the user from exiting it and using the rest of the site.

We could always extend the spec (now or later on) to add a way for apps to request full screen / pip? (but always with the understanding that it wouldn't be guaranteed)

Also, I figure apps can always do stuff like show a dialog within the app that suggest to the user to switch to full screen