But the moment you wanted something like: there's two buttons, one following the other in the bottom right corner of the window, with a certain fixed spacing between them, but otherwise dynamically sized to content, it all broke down. And this just happens to be one of the most simple scenarios, just a basic dialog box with "OK" and "Cancel"!
How did it work in practice? We just made widgets "wide enough" to fit anything that could conceivably be thrown at them. If later that assumption was proven wrong - e.g. because translators came up with a very long string for the label - then the developers would have to go back and redo the UI.
Keep in mind it was usually always the case you could set widget sizes (or do anything else you wanted) "in code", also.
It's not as though you were ever forced to always use the visual designer for absolutely everything.
In general, there are no significant differences whatsoever between the way something like React actually works and the way something like WinForms works.
Likewise, Windows Forms table layout managers and Swing layouts, while not as powerful, did the job.
(WinForms designer technically supports them. But it's less "drag and drop", and more like "drag and ... um, what the hell is this thing doing there now?").
The only problem is how buggy VS designer in some releases tends to be, forcing to restart VS from time to time.
But that affects all kind of stuff, including apps not using layout managers.
And regarding Motif, I surely recall the GUI designers being relatively good.
Likewise with Java Swing and designers like Netbeans Matisse.
Matisse is the only UI designer that I know that does true drag and drop (letting you position widgets exactly where you want them) while also producing flexible layout. And IIRC they had to write a custom Swing layout manager for that.
Sun prototyped it on Netbeans, made it open source and then when everyone was happy, it became yet another layout manager available in any Java compliant platform.
That is the whole point of a layout manager engine, they are extensible.
Which is why project Houdini is having a layout engine APIs as well.