Form restoration interop

[annevk]

As discussed in https://bugzilla.mozilla.org/show_bug.cgi?id=654072 it might be good to have more alignment about what form restoration entails. Questions that came up:

• Is disabled restorable state?
• Should restoration happen on "soft" reload?
• Should there be an event? It sounds like pageshow might be good enough for this, provided things are restored at that time. As noted in Clarify that form state restoration does not fire events #7283 it seems developers could also use formStateRestoreCallback for this, which makes it very observable when exactly this process takes place...

Further context: #6853 (comment).

cc @gijsk @whatwg/forms

[calebdwilliams]

Not sure if there has been any movement on this that hasn't been tracked on here specifically, but I would argue for

Is disabled restorable state

that for the custom element reaction, that specific state should be opt-in-able via the state previously set on ElementInternals.prototype.setFormValue since that method accepts both a form value and a state. Although it appears as if even Chrome is no longer calling formStateRestoreCallback with the previous state, only the value(warning: overly-simplistic demo).

What I'm confused about (and maybe this is a separate issue) is how, exactly, the formStateRestore reaction should be triggered in the case of autocomplete. This is beginning to be an issue for the teams I support. If this is the right place to discuss that behavior, let me know; otherwise, I'd be happy to create a new issue.