Kwin builders, please read

If you build kwin master from scratch stray files from old builds can
cause crashes.
We've had a few duplicate bug reports of this, so I'm sending an email.

The most important file to remove is.
$prefix/lib/qt5/plugins/org.kde.kwin.waylandbackends/KWinWaylandWaylandBackend.so

You can also safely remove:

$prefix/lib/plugins/kf5/org.kde.kwindowsystem.platforms/KF5WindowSystemKWinPrivatePlugin.so
$prefix/lib/plugins/kf5/org.kde.kglobalaccel5.platforms/KF5GlobalAccelPrivateKWin.so
$prefix/lib/plugins/kf5/org.kde.kidletime.platforms/KF5IdleTimeKWinWaylandPrivatePlugin.so

This code is now bundled inside kwin, as it is very kwin-specific.

If you run unit tests, please clean the build dir.

Distro packages are unaffected.

David