• Fixes Engine freeze when switching between SceneCollection and AtlasSource in TileMap tab #114523
• The infinite loop in general is actually caused by Allow SplitContainer to have more than two children #90411,
  • Fix distraction-free mode changing split offsets #113822 just makes it easier to trigger

The checks in SplitContainer::_set_desired_sizes fixes the infinite loop.
The problem was the available space / shrink amount got to a small number like 1.2e-5 where is_zero_approx didn't catch it but subtracting it resulted in no change. There is now a check to make sure that the available space changes. There is another check in the similar loop below it since it can have the same issue.

Unrelated to the inf loop, the split offset was being changed when changing from a scene collection source to an atlas source.
This happened because the atlas was being shown before the scene collection was hidden, which made the SplitContainer try fit all 3 children, shrinking them, before it went down to 2 visible children. Since the order matters now I changed it to show only after hiding.