develop.element.io freezes while opening rooms

[bbhtt]

Steps to reproduce

Since today, this wasn't happening yesterday

1. Open develop.element.io
2. Click on a room, in my case #matrix:matrix.org seems to reliably reproduce the issue
3. Wait for a bit as it loads the timeline
4. See that the entire chat timeline freezes and after a while it unfreezes itself

Outcome

What did you expect?

Should not freeze

What happened instead?

Freeze

Operating system

Fedora 39

Browser information

Firefox Nightly 119.0a1 (2023-08-30)

URL for webapp

https://develop.element.io/

Application version

Element version: 1ada110-react-45094bda7c00-js-5ddd453699a3 Olm version: 3.2.14

Homeserver

matrix.org

Will you send logs?

Yes

[bbhtt]

Logs (change to .tar): rageshake.txt

Profiles:

• https://profiler.firefox.com/public/n5g2q2d5c1w6dzzb7ascvx2yxxjzbh7nbxv03c0/
• https://profiler.firefox.com/public/9pa5a4y7w03av2sq4ysqktvt0jmz869d98m3kp0/

I can see it happening in Firefox 117 too: https://profiler.firefox.com/public/akz1c3ywxyt22h00jbgnxyk97nxvxr4fgcv1je0/

Element stable seems unaffected.

Screenrecord:

ccef11d646.webm

Sometimes the debug script option appears in Firefox while in freeze which shows this:

[germain-gg]

Fixed by matrix-org/matrix-react-sdk#11507
Specifically matrix-org/matrix-react-sdk@4d67458

EDIT: Similar symptoms, but different cause

[t3chguy]

Regressed by matrix-org/matrix-react-sdk#11475

[t3chguy]

Affects users without the labs flag enabled

[andybalaam]

I have attempted to address the most urgent thing, which is that this experimental code accidentally affected everyone. My change for that is: matrix-org/matrix-react-sdk#11513 . When that is merged we can remove the Regression and Release-Blocker tags.

I have asked @charlynguyen to look into fixing the actual problem, and given some more details here: matrix-org/matrix-react-sdk#11475 (comment)