For the brief time I used Windows 11 the amount of times I placed a window over another and then clicked on the wrong window because I couldn't tell at first glance where one started and the other ended was absolutely ridiculous.
I'm afraid that the core of the problem is something far more simple and fundamental.
The people designing desktop apps today simply never learned the conventions that make desktop applications good. They grew up with smartphone apps, web apps, electron apps, games, etc.
In fact, you can observe from things like JavaFX, Flutter, WPF, etc., that the trend has long been about the ability of easily creating custom widgets like you could with Javascript (or Flash), rather than the convenience of having a library of widgets that look and feel exactly the same as every other widget in the system.
"I couldn't tell at first glance where one started and the other ended "
Sometimes I am starting to feel like how my dad looked many years ago when I tried to teach him how to use Windows. He simply couldn't see the window borders. With the latest designs I am reaching this point too. I am struggling moving and resizing windows because I can't tell where the border is.
> look and feel exactly the same as every other widget in the system
Which is what? Windows natively has like 4 official looks. You can click around the 2 (!) settings programs and pop open windows for basically every framework windows has created (and deprecated) in the last 2 decades.
> couldn't tell at first glance where one started and the other ended
This was even worse in an RDP session. No drop shadows. I'm not sure who thought "everything should be flat and white" was a good idea.
> I'm not sure who thought "everything should be flat and white" was a good idea.
It's just the old Windows 2.0 look.
Windows 2 had plainly visible borders, with decent contrast depending on your colour settings, so you could see what ended where.
The UX designers copied the look, minus the colors, and without functionality. Whoever thinks, that an 1px border for a resizable element on a 4k display is ok, is insane.