In the split-view sidebar layout (macOS, and now iPad and the iPhone
Duo), the detail NavigationStack that a selection change replaces
writes an empty path through its binding on the way out, so going to
another section and back always landed on the section's root. macOS
already behaved this way before the router.
Only the selected section's stack is on screen and can pop, so path
writes for any other section are ignored.
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