After discussion on another topic (having new sub-windows open in fullscreen or not), I’ve set my Scrivener to open in fullscreen from the start:
This makes all windows stay and open in maximised mode, but it has an unexpected result. The opening dialog of Scrivener is maximised, but unlike other windows, it doesn’t respect the windows taskbar:
Which means all buttons are hidden under the taskbar.
At the same time, since the window itself is normally NOT possible to be maximised (since it’s a control/dialog, whatever you call it), it can’t also be un-maximised:

It CAN be resized, yes, but it’s hard to catch the bottom-right corner when it’s covered by the taskbar

I moved my taskbar around AND used “Size” from the menu, but not everyone remembers that the menu actually exists, so maybe this would warrant looking into.