1667521 - Firefox 81: IntersectionObserver breaking changes for WebComponents

Status: RESOLVED DUPLICATE of bug 1665447

(Core :: Layout, defect)

Description

[jakewski]

Attachment: repro.html

User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/85.0.4183.121 Safari/537.36

Steps to reproduce:

Create a vertical list of simple web WebComponents and observe them with an IntersectionObserver with options: {root: null, threshold: 0}.

See attached HTML file for demo. I added css to make the difference obvious.

Actual results:

In Firefox 81, only about half of the WebComponents in the viewport trigger the observer with the correct isIntersecting property.

Tested Firefox 80, Firefox 81, Chrome 85, and Safari 14. Firefox 81 has drastically different behavior. In the attached HTML file's comments (L50-L53), there is an equivalent example with normal HTML elements, in which Firefox 81 behaves as expected.

Expected results:

All WebComponents in the viewport should be intersecting with the root node in Firefox 81.

Comment 1

[Erik Nordin [:nordzilla]]

Hi jakewski,

Thank you for reporting this.

I followed your instructions and it looks like this behavior regressed in this patch:

https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=c34351a5fd6c6bfb76ffd7b86b8757d6d95b581c&tochange=8992f6cfc32358bd6b3494d6507911530951a653

But, thankfully, things appear to have been fixed by this patch:

https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=c8ffb751d7d008a07a34b02239d5f542d456ebfe&tochange=92f24022a62c77f83ec94de85aeb9e1a520251de

I am going to mark this as resolved, for now, as the fix should be released in Firefox 83.

Please feel free to re-open this bug if needed.