Regression since #2100

When removing the last file in a grouped or ungrouped open folder it still shows as opened.

No need to address that; it's an edge case that will likely be difficult to resolve and will have a high blast radius.

I had a brief look at earning the bonus points but it looked like it would take me a little bit of time to figure what would be a suitable variable name to replace it (in some of the places). I can open a pull request for it at some point if noone else does it first.

Also I was expecting there might still be some edge cases this didn't handle but I was thinking maybe some of those edge cases might be caused by a similar issue to this one, where the state of each node in a group should be changed rather than a single node (but I'm not at all certain about that because I haven't found buggy edge cases or looked at the code for stuff).

I had a brief look at earning the bonus points but it looked like it would take me a little bit of time to figure what would be a suitable variable name to replace it (in some of the places). I can open a pull request for it at some point if noone else does it first.

No worries, well out of scope.

Also I was expecting there might still be some edge cases this didn't handle but I was thinking maybe some of those edge cases might be caused by a similar issue to this one, where the state of each node in a group should be changed rather than a single node (but I'm not at all certain about that because I haven't found buggy edge cases or looked at the code for stuff).

That's fine. Folder/grouping is complex and we can only fix what we know about.