I'm using Samsung DeX (on a Samsung Galaxy Tab S6), which gives a desktop like experience and makes use of Android windowing support.

Samsung has documentation on how to enable window resizing for Android applications.

This documentation also has more tweaks and information on how to improve the desktop experience in case you're interested.
They also have an app testing guide (although some apps are fine with this guide but won't work on "real" DeX sometimes).

At the moment I can't even start KDE connect within Samsung DeX.

I would happily test an APK if you want me to test one.

PS: I know that it is possible to _force_ all apps to be resizeable via Samsung Labs > Force Resize but it would obviously better to make the application natively resizeable to create less friction.
PPS: I'm not sure whether this is relevant but this is how GitNex fixed the issue and this is how NewPipe fixed it.
PPPS: There's also this blog post that looks helpful.