Kwin hangs in KWin::WindowPixmap::create() / xcb_request_check

After several days using the KDE Plasma desktop, I suddenly found that all of the windows had become unresponsive. I could still move the mouse, but clicking didn't do anything. I could no longer change the volume with the volume buttons. Also, the panel clock stopped ticking upward. Fortunately, I could still use the keyboard to access a tty.

I would estimate this occurred roughly monthly.

I still cannot consistently reproduce, and I wasn't doing anything unusual these interruptions occurred, just switching windows or typing in a textbox.

This normally works from a tty:

    DISPLAY=:0 xdpyinfo

but during these episodes it would just hang.

I believe this would also hang after running and switching back to the graphical window:

    DISPLAY=:0 qdbus org.kde.KWin /KWin supportInformation

But I don't recall for sure.

This also hung:

    DISPLAY=:0 kwin --replace

and showed an error:

    qt.qpa.screen: QXcbConnection: Could not connect to display :0

Next tried this:

    DISPLAY=:0 kquitapp5 plasmashell && kstart5 plasmashell

Got a different error after a timeout:

    "Quitting application plasmashell failed. Error reported was:\n\n org.freedesktop.DBus.Error.NoReply: Did not receive a reply. Possible causes include: the remote application did not send a reply, the message bug security policy blocked the reply, the reply timeout expired, or the network connection was broken."

In a later instance, I was able to revive the system like this:

    kill -USR1 $(pgrep kwin_x11)
    DISPLAY=:0 kwin --replace

I found this sufficiently convincing that is was an issue with Kwin and not another component, so I thought I should report this bug. Backtraces and strace logs are attached, as is my ~/.config/kwinrc and the output of the "qdbus org.kde.KWin /KWin supportInformation" when it is in a responsive state.

$ lsb_release -rd
Description: Ubuntu 18.04.4 LTS
Release: 18.04

Kernel Version: 5.3.0-61-generic

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: kwin-x11 4:5.12.9.1-0ubuntu0.1
ProcVersionSignature: Ubuntu 5.3.0-61.55~18.04.1-generic 5.3.18
Uname: Linux 5.3.0-61-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.15
Architecture: amd64
CurrentDesktop: KDE
Date: Sat Jul 4 16:13:00 2020
InstallationDate: Installed on 2020-02-01 (154 days ago)
InstallationMedia: Ubuntu 18.04.2 LTS "Bionic Beaver" - Release amd64 (20190210)
SourcePackage: kwin
UpgradeStatus: No upgrade log present (probably fresh install)