With mesa 26.2 from experimental and as of kwin 4:6.7.4-1 I'm seeing Wayland
clients disconnecting (i.e. crashing) when making surfaces visible. Journal
says:

kwin_wayland_wrapper[2472]: error in client communication (pid 4139)
firefox-nightly[4139]: [4139] WaylandProxy: wl_display.error: the given registry did not announce global 66
firefox-nightly[4139]: [GFX1-]: (kde) Wayland protocol error: wl_fixes#68: error 0: the given registry did not announce global 66

kwin_wayland_wrapper[2472]: error in client communication (pid 2653)
polkit-kde-authentication-agent-1[2653]: wl_fixes#43: error 0: the given registry did not announce global 66
polkit-kde-authentication-agent-1[2653]: The Wayland connection experienced a fatal error: Protocol error

This seems to be due to the backporting of wl_fixes v2 in 6.7.3 upstream:
https://invent.kde.org/plasma/kwin/-/commit/8b8d2b24d1298dffd5f7c7f8cd384cce466752ea

Vlad Zahorodnii recommends backporting another upstream commit to fix this:
https://mail.kde.org/pipermail/distributions/2026-August/001725.html

Regards,
Simon

PS Ignore the kwin version in my system info, I downgraded to stop clients
crashing.