This is a feature request and a follow up to the internal bug 21870326.

I propose the following behavior:

1. When casting starts, the user visible controls should sync to the remote state.
2. While casting, the controls should control the remote state only and follow the separate mute/volume behavior (muted doesn't set the volume to 0, so unmuting keeps the volume at the previous level). Volume up should probably unmute and volume down shouldn't (but I think it's up to the Cast device to do that).
3. When casting stops, we should sync the volume to the previous setting (so we should keep it at pt 1) -- similar to how device volume keeps different values for different output sinks like when I plug/unplug my headphones.

That'd mean that while playing remotely the hardware/keyboard volume commands would affect the visible media element's volume though which IIRC they don't during the local playback. Making those work is tricky though if the user is playing multiple elements - should the volume for all be affected?

While at it, I'd also argue that we probably shouldn't handle headset commands when casting anything but it's probably for a separate bug.

+Mark, Yuri for desktop inspired thoughts :)