Avoid creating redundant containers when switching between tabbed/stacked and split layouts

[orestisfl]

Fixes #3001

[stapelberg]

I don’t have time to look at this in detail right now, but can you outline why tree_flatten in src/tree.c is not sufficient? I would have expected a fix to land there.

[orestisfl]

I don't think single-parent containers are inherently bad that's why it's hard to find a general rule that would fix the issue and keep tree_flatten's behavior acceptable in all cases where it's called.

[orestisfl]

@stapelberg Any chance we re-visit this PR? I feel it's one of the major causes of pain for users. I think including a fix in tree_flatten is not the way to go. The function cannot always be called except for "cleanup" operations after moves etc and I feel that it is much cleaner to prevent creating a situation rather than fixing it afterwards.

[stapelberg]

OK, I’ll try to take another look in the coming days.