Bug 506782

Thanks for the debugging!

If KWin has high CPU usage as well, most likely plasmashell is presenting way more often than it should, and processing those commits just takes time.

There was an issue where Qt would attempt to run animations on hidden windows and go crazy because of that. This sounds like it could be a symptom of that same problem.
Do you have a panel set to auto-hide perhaps, or some non-default widgets?
Can you also reproduce the problem with a new user?

I do have a Latte-style dock set to auto-hide. I can try messing around later to see if that's the trigger - I do recall seeing icon updates from the dock in a stack trace at some point, though I'm not sure if that was the same issue.

Okay, do try to set it to always visible. If you can check if it happens with a new user, that would be the most safe option.

Okay, bear with me here - I forgot what the original issue was until I just re-read it after experiencing a hang after unlock, which I got into my head as the issue. However, I think it may have some insight as to the source of this issue.

I disabled my auto-hiding panel, and today unlocked to find plasmashell hanging again. I already had GDB attached in anticipation of this issue, which I promptly used to break into plasmashell and see its stacktrace. I found that the global app menu I have on one of my panels appears to be taking a long time in the stack trace shown below:

#0  0x00007f4f04b3860d in syscall () at /usr/lib/libc.so.6
#1  0x00007f4f0555a9b1 in ??? () at /usr/lib/libQt6Core.so.6
#2  0x00007f4f05390f93 in QMetaObject::invokeMethodImpl(QObject*, QtPrivate::QSlotObjectBase*, Qt::ConnectionType, long long, void const* const*, char const* const*, QtPrivate::QMetaTypeInterface const* const*) () at /usr/lib/libQt6Core.so.6
#3  0x00007f4f063f5c34 in ??? () at /usr/lib/libQt6DBus.so.6
#4  0x00007f4f063f6817 in ??? () at /usr/lib/libQt6DBus.so.6
#5  0x00007f4f053e78bc in QObjectPrivate::connectImpl(QObject const*, int, QObject const*, void**, QtPrivate::QSlotObjectBase*, int, int const*, QMetaObject const*) () at /usr/lib/libQt6Core.so.6
#6  0x00007f4f053e7c44 in QObject::connectImpl(QObject const*, void**, QObject const*, void**, QtPrivate::QSlotObjectBase*, Qt::ConnectionType, int const*, QMetaObject const*) () at /usr/lib/libQt6Core.so.6
#7  0x00007f4e640cc1d1 in ??? () at /usr/lib/qt6/qml/org/kde/plasma/private/appmenu/libappmenuplugin.so
#8  0x00007f4f053e99d4 in ??? () at /usr/lib/libQt6Core.so.6
#9  0x00007f4ede05798b in ??? () at /usr/lib/libtaskmanager.so.6
#10 0x00007f4f053e99d4 in ??? () at /usr/lib/libQt6Core.so.6
#11 0x00007f4f056463b2 in QIdentityProxyModelPrivate::sourceDataChanged(QModelIndex const&, QModelIndex const&, QList<int> const&) () at /usr/lib/libQt6Core.so.6
#12 0x00007f4f053e99d4 in ??? () at /usr/lib/libQt6Core.so.6
#13 0x00007f4f0560fb84 in QAbstractItemModel::dataChanged(QModelIndex const&, QModelIndex const&, QList<int> const&) () at /usr/lib/libQt6Core.so.6
#14 0x00007f4ede075fe7 in ??? () at /usr/lib/libtaskmanager.so.6
#15 0x00007f4f053e99d4 in ??? () at /usr/lib/libQt6Core.so.6
#16 0x00007f4ede070951 in ??? () at /usr/lib/libtaskmanager.so.6
#17 0x00007f4f063a62ee in ??? () at /usr/lib/libffi.so.8
#18 0x00007f4f063a29f8 in ??? () at /usr/lib/libffi.so.8
#19 0x00007f4f063a56ce in ffi_call () at /usr/lib/libffi.so.8
#20 0x00007f4f0807547a in ??? () at /usr/lib/libwayland-client.so.0
#21 0x00007f4f080762e9 in ??? () at /usr/lib/libwayland-client.so.0
#22 0x00007f4f080766bb in wl_display_dispatch_queue_pending () at /usr/lib/libwayland-client.so.0
#23 0x00007f4f064d6f36 in ??? () at /usr/lib/libQt6WaylandClient.so.6
#24 0x00007f4f053ea001 in ??? () at /usr/lib/libQt6Core.so.6
#25 0x00007f4f05683109 in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) ()
    at /usr/lib/libQt6Core.so.6
#26 0x00007f4f05387d46 in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) ()
    at /usr/lib/libQt6Core.so.6
#27 0x00007f4f0537db7c in QCoreApplication::exec() () at /usr/lib/libQt6Core.so.6
#28 0x000055f8fdb5ff78 in ??? ()
#29 0x00007f4f04a27b8b in __libc_start_call_main
    (main=main@entry=0x55f8fdb5cf30, argc=argc@entry=2, argv=argv@entry=0x7ffec59a0258)
    at ../sysdeps/nptl/libc_start_call_main.h:58
#30 0x00007f4f04a27c4b in __libc_start_main_impl
    (main=0x55f8fdb5cf30, argc=2, argv=0x7ffec59a0258, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7ffec59a0248) at ../csu/libc-start.c:360
#31 0x000055f8fdb60495 in ??? ()

This leads me to believe that the KWin hang may be tangentially related to the app menu constantly updating, and those updates getting stuck in a queue while KWin does... something. Now that I remember that I don't have to wait 24 hours for the plasmashell hang, I'll follow the reproduction steps I gave myself (:P), and see what's going on in a separate user. I'll also upgrade to Plasma 6.5 to see if the issue got fixed in the process.

By the way, the monitor turning back on issue has gone away since Plasma 6.4.4 or 6.4.5.

Disregard the above. I can confirm the issue goes away when I disable auto-hiding on the dock. I can also confirm that when the panel is set to Dodge Windows, the issue only appears when it is hiding, and doesn't happen when there are no windows to dodge. The dock panel only contains a standard Icons-Only Task Manager, with some spacers on the sides. I'm currently upgrading my system, and while I won't be on Plasma 6.5 yet (didn't realize it's still in Arch testing), I'll be able to test with Qt 6.10 in case that fixed the issue.

Qt 6.10 appears to have solved this issue for me. I'm closing it for now, but if I find it again in the future I'll reopen.