Etienne,

This editing to QgsCollapsibleGroupBox may affect the layout for them in the Oxygen theme (which I can't test here). Can you verify?

Also, the new app stylesheet will now cause a collapsed (flat) QgsCollapsibleGroupBox to not show a line under it (i.e. the border should now be 0% alpha), if the new 'Use custom group boxes' is checked under Options-->General section, Application group. I think this looks better than any line showing up (which varies a lot between OSes).

Thanks