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.