How about the performance here? I'd think that OS-managed applications are faster, as the OS can use the GPU for overlaying (But no idea).
GDI hardly uses GPU. Since Vista the desktop is doublebuffer on the OS level though and the composition of those double buffers uses GPU.
In general, I wouldn't worry about that. Modern UI is mostly empty monochromic coloured space, that is fast on nearly everything. Antialiased font rendering is usually the most expensive of GUI anyway nowadays, not straight painting.
Modern UI was developed in the hope that one single application would work for various screenformats (PC- Tablet - telephone) without designing screens for every application.
In practice, most mobile screens are still manually designed though, to make the most of the available surface area. (though maybe that will change in time since mobile resolutions are growing fast)
Is Java doing it the same way (self-managing its application window)?
Swing did. I believe AWT tried to be a bit more
Note that Delphi's Windows 10 controls are modelled after their UWP app counterparts but are afaik also for quite a large part ownerdrawn. Ownerdrawn within the LCL/VCL framework though, not a 3rd party library with its own philosophies like MSIDE.