Bug 309699 – ResizeObserver retains references to removed DOM elements without explicit unobserve() resulting in poor performance - they are GC'd in Firefox and Chrome

• Status: NEW
• Component: WebCore Misc.

[Paul Rhomberg]

Created attachment 478631 [details]
Minimal reproduction

Resize observers don't seem to garbage collect effectively. When observing a lot of elements and removing these without calling .unobserve() on them, performance degrades. 

To illustrate this issue, the provided html file does the following:
1. On button click, it creates 10,000 elements
2. It observes them all with a ResizeObserver
3. When the button is clicked again, the elements get removed and new ones get created & observed

While on Chrome and Firefox, this cycle can be performed hundreds of times without slowdown, on Safari, the page starts lagging drastically after ~10-20 cycles, and each cycle takes longer and longer.

This indicates that Firefox and Chrome perform some sort of garbage collection even when .unobserve() is not called, whereas Safari does not.

Tested on Safari 26.3.1 on iOS, iPhone 15 Pro

[Radar WebKit Bug Importer]

<rdar://problem/172597260>

[Karl Dubost]

Created attachment 478702 [details]
rendering of timelines in webinspector for multiple clicks.

Thanks Paul. This is indeed happening.
We can indeed see how the click event takes longer and longer to get dispatched.

[Karl Dubost]

To note that the ResizeObserver callback is pretty constant around 0.113ms

[Ryosuke Niwa]

https://commits.webkit.org/309376@main should have addressed this issue for most part.