Sounds like QML, which even looks slightly like JSON if you squint at it right. Bonus (depending on your views) points: you can even use javascript in it.
Every QML application I've tried was comparable to Electron in my eyes so far: slow and full of little usability bugs.
This started to be really noticeable for me in the linux world, where QT is already common, but more and more UIs are converted to QML from QtWidgets.
The difference is staggering for me. QML GUIs are also often styled by the authors, which very often break or replace the system theme, something I hate.
I'll admit to completely ignoring the system theme, but that's absolutely standard practice among photo editing software like RawTherapee, darktable, and proprietary ones like Lightroom and Capture One.
As far as slow, no way. Instant startup, instant everything, no memory leaks. I've left my editor running for multiple weeks straight with no issues.
Usability bugs? Yes, there's basically one: scrolling. I've hacked together my own momentum scrolling system but I'm not happy with it. At least it's better than scrolling in KDE Plasma native applications, though.
RawTherapee has the option to use the system theme, and I'm quite grateful for that.
darktable on the other hand has basically one dark theme only which also shows stupid UI mistakes as black-overscroll-on-black-background in all lists and fixed font size. If there is ONE setting you have to keep, is the FONT SIZE in the system theme!
You're excluding many more issues from here. Click on a scollbar and drag outside: very often the scrollbar doesn't release when the mouse button is depressed, though styling and custom events play a role. QML is not immune to a lot of quirks in highdpi, most of which are completely absent in QtWidgets. In detail, font rendering (hinting) in QML in GL contextes is basically broken.
The whole UI is perceptibly laggier as a whole. I'm a programmer myself. As a programmer I feel ambivalent, but as an user, I hate it.
It seems like any consensus on ignoring/utilizing system themes is transient. I know I'm glad when they are mostly ignored on mac, while glad when they are utilized on linux.
