Sometimes a frame node can't be behind another because of parenting.
Comparing the two nodes with `compare_node_depth` would give that
result too, but sometimes every combination of frame node isn't tested.
In that case, the existing values of `ui_order` matter, and we should start
the sort there. So first sort based on the existing order, then do a stable
sort to fulfill the constrains coming from selection and parenting.
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