It is possible to create CloseWatchers for dialog elements and popovers which aren't in fully active document

[smaug----]

What is the issue with the HTML Standard?

One can't create CloseWatchers for non-fully-active document through the constructor
https://html.spec.whatwg.org/#dom-closewatcher, but can using dialog element or popovers
https://html.spec.whatwg.org/#dom-dialog-showmodal step 9
https://html.spec.whatwg.org/#the-popover-attribute:establish-a-close-watcher 11.8

@domenic @keithamus

[domenic]

Note: there are ~2 types of non-fully-active documents. The first is a document in bfcache. The second is one without a browsing context, e.g. created by new Document(). This issue is about the latter, because you can't run JS in the former.

I wonder if it'd be web-compatible to throw for showModal() in non-fully active documents. It's pretty nonsensical anyway.

But, the simplest path is to probably just no-op the close watcher step.