SUMMARY
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STEPS TO REPRODUCE
1. Start KDE/Plasma with Wayland.
2. Press "New Message" in Kontact (in the Kmail view).
3. Change "To", the "Subject", or add some message text.

OBSERVED RESULT
When adding a new mail as "To", the GUI draws a new line, which overdraw the "Subject" line, so that is is not visible anymore.
When typing something in the subject line, the Window title changes accordingly, but the text in the subject field remains invisible.
When adding a message (I use the KMail plugin to call an external editor), the external editor opens, I type the message, close the window, and nothing happens.

All of the above GUI issues fixes itself when resizing the window or pressing "Send->Abort". So I'm pretty sure somewhere is a missing repaint.

EXPECTED RESULT
The GUI updates immediately.

SOFTWARE/OS VERSIONS
Windows: -
macOS: -
Linux/KDE Plasma: Gentoo Linux with Plasma 5.27.4
(available in About System)
KDE Plasma Version: 5.27.4
KDE Frameworks Version: 5.104.0
Qt Version: 5.15.8

ADDITIONAL INFORMATION
This bug happens only on Wayland. Forcing the program to xwayland with `QT_QPA_PLATFORM=xcb kontact` also fixes the problem.
This bug is not new, I experienced it at least since last summer, so two(?) Plasma releases ago.

Maybe related to https://bugs.kde.org/show_bug.cgi?id=424024