Interesting...
At first, it's great you think about sessions and closing them. And I agree with you that the list of all (sub)models grows quickly and does not work well as an overview then.
Currently I use models in LDCad in this way:
1. I try to open only one mpd file in LDCad
2. Then the list of submodels accessible from the top-right navigation is good to see submodel names - it is helpful and well readable, comparing to images of submodels, cca 10 milimeters wide, in the Part Bin.
3. One may argue that I may make those images bigger but this size is OK for images of Parts and I need to save space on the screen
4. if I have more than one MPD open and want to switch between them, I use the top menu: Model->Change current model. But the list is too long now. "{M}" marker helps a little.
Therefore I'd prefer:
1. "session" is always per MPD file - I do not want to close a submodel only
2. top menu -> Model -> Sessions showing a list of sessions == MPD files. Similar to "Windows" menu in another GUI applications
3. the top-right navigation containing a list of submodels in the current model only - in the end, this navigation is placed inside the Editing window and so I expect it to be relative to the current MPD open in that Editing Window
4. closing a session should be equal to closing the MPD (i.e. while closing LDCad itself). This way there will be no confusion.
I'm not sure, Roland, I understood your description, so I let you to compare our ideas