> There isn’t a fully general solution to positioning and sizing GUI boxes.

flexbox. You can literally recreate any fully responsive desktop style GUI with it. You just need to layout your elements the same way you would in a GUI designer like QtCreator or Glade. Which means thinking in terms of divs with flex-grow set on them to get the automatic container expansion you get with "vertical boxes" or "horizontal boxes". These divs will sometimes be empty if you want a simple "expander" box.

Once you have it down you can look at any desktop GUI and just inherently understand how it maps into this flexbox model.