Created attachment 441033 [details]
Sample of Safari Web Inspector process
Steps to reproduce:
1) Open a private Safari window
2) Open https://twitter.com/jack
3) Open the web inspector
4) Select the Elements tab
5) Scroll down the Twitter page
This bug occurs reproducibly with both Safari Version 15.0 (166220.127.116.11.4, 16612) and Safari Technology Preview Release 133 (Safari 15.4, WebKit 16618.104.22.168) on macOS Version 11.6 (20G165).
Attached are samples of the hangs. I'm also seeing the web inspector process at >100% CPU in Activity Monitor.
Created attachment 441034 [details]
Sample of Safari Technology Preview Web Inspector process
Thank you for the report! This issue appears to no longer reproduce with a top of tree build, but does reproduce for me in STP 133. I believe this was fixed by r282607 (and possibly also improved by other related performance/hang fixes r282475, r282477, and r282610 related to DOM mutation), all of which are just past the range of fixes in Safari Technology Preview 133.
Thanks. I'll test with the next Safari Technology Preview.
I just tested with Safari Technology Preview 140 (Safari 15.4, WebKit 16622.214.171.124), and this bug does indeed appear to be fixed. Thanks!