Fixes regression from #7523
I messed two things up around spatial navigation in the split tree
that this commit fixes:
1. The distance in the spatial tree only used a single dimension
that we were navigating. This commit now uses 2D euclidean
distance from the top-left corners of nodes. This handles the case
where the nodes are directly above or below each other better.
2. The spatial slots include split containers because they are layout
elements. But we should only navigate to leaf nodes. This was
causing the wrong navigatin to happen in some scenarios.
