What should happen to hover, focus and active chains when the flat tree changes?

[emilio]

All browsers have similar invalidation issues:

• https://bugs.chromium.org/p/chromium/issues/detail?id=939769
• https://bugzilla.mozilla.org/show_bug.cgi?id=1551621
• (Not sure if there's a WebKit bug).

See the test-case in the chromium issue for an example.

I proposed to handle it in Gecko the same way content removal is handled, but @smaug---- disagreed. I think it's simpler to do it that way since then you don't need to differentiate when you're getting re-assigned from when you're getting assigned to an unexisting slot for example.

Anyhow, should the focus be kept in the same element silently? Or should the focus be removed the same way it happens for content removals? Same for :active.

cc @lilles @rniwa @annevk

[lilles]

I think I tend to agree with @smaug----.

One case where I'd be surprised to lose focus: if you attach a shadow root to an element in your ancestor chain and just add a slot to it. No rendering changes, but focus lost?

With my implementor hat on, handling it as a DOM remove/add is simpler, of course.

[rniwa]

if you attach a shadow root to an element in your ancestor chain and just add a slot to it. No rendering changes, but focus lost?

Yeah, this would be indeed a surprising behavior. I tend to agree with @smaug---- here that the expectation is to keep focus, hover, active, etc... states intact.