This KCM ships with a set of thumbnails that are used when one wants to test a given window switcher: dolphin.png, falkon.png, kmail.png, systemsettings.png The problem is that it then looks for the default handlers for mimetypes related to these PNGs: respectively "inode/directory", "text/html", "message/rfc822" and "kdesystemsettings". If one has configured different handlers for some of those mime types, it is easy to end up in a situation where the thumbnail clearly shows KMail but the title says "Thunderbird", or there's a Falkon thumbnail accompanied by a text saying "Firefox".
A possibly relevant merge request was started @ https://invent.kde.org/plasma/kwin/-/merge_requests/7065
Git commit fafdf099cdf6b323d3cf41111fd802e7d0dbe677 by Nate Graham. Committed on 27/01/2025 at 23:41. Pushed by ngraham into branch 'master'. kcms/tabbox: hardcode app names and icons in the preview Right now the thumbnails are hardcoded, but the app names and icons are not, and change based on the user's default apps. This is nonsensical; the thumbnail won't match what the name and icon claim the app to be. Let's stop doing that and instead just hardcode the names and icons to match the thumbnails. FIXED-IN: 6.4.0 M +17 -13 src/kcms/tabbox/layoutpreview.cpp https://invent.kde.org/plasma/kwin/-/commit/fafdf099cdf6b323d3cf41111fd802e7d0dbe677