Close watchers

[domenic]

Request for Mozilla Position on an Emerging Web Specification

• Specification Title: Close Watchers
• Specification or proposal URL: https://wicg.github.io/close-watcher/ (explainer: https://github.com/WICG/close-watcher)
• Mozillians who can provide input (optional): maybe @annevk and @smaug---- since it's kinda history-adjacent, and I believe we've discussed it briefly before?

Other information

We've been prototyping this API over in Chromium for some time. It's a small offshoot of the work in #543 on new history APIs generally. The original intent was to make it so that developers don't have to abuse the history API to detect Android back button presses, and this is still probably the largest impact. We've also found that it has some significant benefits in terms of smoothing over platform differences, since it "just works" no matter if you're using a keyboard, Android software back button, Android swipe gestures, or assistive tech gestures.

Previous feedback from @annevk was that we should be sure to integrate with existing primitives such as fullscreen and <dialog>. We took that to heart and I'm pretty happy with how the result turned out; the updates to other specs now put them on a shared foundation that's pretty nice and much more rigorous than previous prose, IMO.

[annevk]

@smaug---- and I had a brief discussion about this with @domenic. We had some minor suggestions, but overall this design looks reasonable to us. I suggest we mark it as worth prototyping. As it's quite small I'm not sure it warrants a dashboard entry. If anyone has feedback on that or it being worth prototyping, now would be a good time to say something.

(Marking as WHATWG as well because the aim is to upstream this into HTML.)