Using Qt WebEngine can completely freeze the UI.
It seems to be caused by a deadlock; after attaching gdb the main thread is blocked inside QBasicMutex::lockInternal()

I was able to reproduce with

• 6.9.3 and 6.10.0 official releases
• a custom release build of 6.9.3
• a custom release build of Qt WebEngine 6.9.3 build against Qt 6.9.2

I was not able to reproduce with 6.9.2

It is possible to reproduce the issue with the simplebrowser demo.
However it takes a bit of playing with the app before it freezes and it occurs a bit randomly.

I was able to reproduce the issue with a developer build.
I found 3 threads blocked on something related to Skia (see attached images), other threads did not seem to be blocked on something interesting.

The only change I found between 6.9.2 and 6.9.3 related to Skia is the fix for QTBUG-138641 : https://codereview.qt-project.org/c/qt/qtwebengine/+/667817

After reverting it locally, I could not reproduce the issue.
However given that the fix is a bit random, I will run more tests later to be sure it is related to this change or if I was just lucky it did not freeze.

I was able to reproduce the freeze with https://codereview.qt-project.org/c/qt/qtwebengine/+/667817 reverted so it is not the root cause of the freeze.
However it is much easier to reproduce with this commit than without it.
With simplebrowser:

• with 6.9.3 I could easily freeze the app under 1min of playing with it
• with 6.93 + the commit reverted it took me more than 5 minutes of clicking on every links to get it

With my own app:

• with 6.9.3, the app freezes in 1 or 2 interactions
• with 6.9.3 + the commit reverted I was not able to reproduce it (yet?)

For now I had the issue on 2 laptops:

1. Kubuntu 24.04
   X11
   Linux 6.14.0-1013-oem
   Intel Core Ultra 7 165H
   nVidia RTX 500
2. Ubuntu 24.04
   Wayland
   Linux 6.14.0-33-generic
   Intel Core Ultra 7 165U