Last time i checked (around 7 years ago) a LO extension that wanted to show some GUIs (dialogs) could only create windows by hand, and then fill them with widgets of fixed dimensions, 90s style. No automatic resizing, alignment or hierarchical box model were available. Is that still the case?
I’ve seen the 2017 FOSDEM presentation about allowing Glade UI files to be used by extensions to construct GUI, but i can’t seem to find any mentions of that in the devguide.