Yeah, I were obsessed a long time ago. It doesn't have to be automatic. Start from what we have in css, there's media query. Looking good on desktop/laptop/mobile seems like special case. So generally we might look for semantic that scales e.g. ratio, open height, open width, .. idk I don't figure it out yet, it's like a greenfield area where previous solution doesn't look like revolutionary.
Traditional desktop ui idiom implies enough space for all controls to fit. Frameworks could include similar width-breakpoint sets of constraints, or e.g. a type of H:a|-|b constraint which could transform into V:a|-|b plus H:parent|-|b when parent is too narrow, essentially wrapping an element.
But they usually don’t. It doesn’t mean that constraint-based layout lacks flexibility. Only specific engines do, but so do specific table engines. It’s easy to forget that css also sucked for 20 years straight, and still does, if you dig an inch deeper than usual. E.g. grid is cool, but did anyone try to page-break it in media print? Can we baseline—align divs & inputs that are not direct children of a flexbox? Not to mention internalized bugs like when your grid gets a parasite scroll when its scrollable child has large padding specified in particular units.
All that said, I think the best toolkit is the one which contains all of the tools and not only a subset.
