Resizing is possible
It's not resizable via dragging borders or corners and the resizing button is greyed out
As you can see the resizing button is greyed out:

I'm using Samsung DeX, which gives a desktop like experience and makes use of Android windowing support.
Samsung has documentation on how to enable window resizing:
<application
android:resizeableActivity="true">
</application>
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).
Applications like Firefox, VLC or Termux can be resized without any problems.
PS: I know that it is possible to _force_ all apps to be resizable via Samsung Labs > Force Resize but it would obviously better to make the application natively resizable to create less friction.
PPS: I'm not sure whether this is relevant but this is how GitNex fixed the issue.
PPPS: There's also this blog post that looks helpful.
Can you test this APK, please:
NewPipe_basic_resize_APK.zip
Hi @TobiGr,
Most helpful comment
Can you test this APK, please:
NewPipe_basic_resize_APK.zip