The Problem
Things that rely on identifying windows can’t differentiate between different Mailspring windows (e.g. the main window and the new email window) by their window class. It looks like “mailspring Mailspring” is used for everything.
Proposed Solution
Use different window class names for different kinds of window. (e.g. “mailspring MainWindow”, “mailspring MessageWindow”, etc.)
Alternative Solutions
I can’t think of an alternative solution.
Context
This means you can’t do things like have separate window rules in KDE for different windows. One of the workarounds for Wayland windows not being able to position themselves is using window rules to place things as you want (e.g. maximising the main window), but this isn’t possible with Mailspring because of not being able to differentiate them.