They have to reflow any time the user resizes their windows.

The whole operating system is even named after that concept.

Yeah, sounds like bullshit. I don't even see why that particular concern would create more work on the OS's part.

If an application fits "wonderfully" into the space it's given, Windows did nothing but telling it the dimensions it needs to fill. And as you said those dimensions can vary wildly.

Yeah especially considering you can install 3rd party solutions to dock the taskbar to the left which work perfectly fine

when Linux desktop environments built by hobbyists

That might be a bit obsolete as a state of things. Like 15 years obsolete.