Both Chrome and Safari now allow the user to change what surface is captured.

That's obviously great stuff. Can we make it better still?

• What if an application intends to do some processing that depends on the captured content?
• What if an application wants to set different constraints, for instance when capturing a window vs. when capturing a screen?
• What if the application intends to save different surfaces to different files, and wants to start appending to a new file whenever the user changes the source?

So I propose that we add two things:

1. A control which allows an application to instruct the browser - whenever the user changes what surface is shared, pause the track (by setting enabled to false).
2. Fire an event whenever this happens. (With the general expectation that the application will apply new processing and then set enabled back to true.)

Possibly the two can be combined, by specifying that setting an event handler signals that the pausing behavior is desired (@alvestrand's idea).

Another natural extension of this idea is to also apply it when a captured tab undergoes cross-origin navigation of the top-level document. When that happens, some applications might wish to stop capture momentarily and (in-content) prompt the user - "do you still want to keep sharing?"

Relevant previous discussion here.