At first sight the situation seems to be a bit challenging: the map would require a map.invalidateSize() on expand, but it seems nothing in the code can notify us about the change in container's collapsed/expanded status...
As a consequence, I'm not sure where to put a hook and ask leaflet to redraw the map, but I'll keep poking my head around it.
We could go a bit further and avoid creating maps that are hidden by default. Only instantiate the Leaflet map when the collapsed container expands. This would save some JS execution time on page load. I will create a ticket in the backlog.
I re-open this task, because there are problems with maps in the collapsible environment again. Check, for example:
and click on "Открыть карту". Only a small part in the top of the map is displayed.