> And doing UI layout on background threads breaks the basic design of pretty much every UI framework, web or native, which are usually single-threaded.
By "single-threaded" I meant business logic and non-fixed layout being done on the same thread. "Rendering" (drawing to a bitmap or translating to the OS compositor's object model or direct OpenGL/DirectX/etc.) is done on a separate thread in most frameworks I'm aware of.
Dunno about the others but Qt has a threaded render loop (http://doc.qt.io/qt-5/qtquick-visualcanvas-scenegraph.html#t...) (and also has no problem handling 1080p/60fps animations on embedded hardware)