resolveTerminalLayoutRoot discarded any known tree that did not cover the
published leaf set exactly and rebuilt the tab as a flat chain with a guessed
'horizontal' direction, restacking side-by-side panes. The guess is then
published, mirrored to every paired client, and written back over the real
tree, so the direction is gone from disk.
Prune a known tree to the leaves that survive and graft only the leaves no
tree places, which is now the sole place a direction is invented and is still
reported through onSynthesize.
Neil
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