The three rebase-onto menu items read Modes().MarkedBaseCommit.GetHash()
(a bare string field) and, on success, cleared it via
ResetMarkedBaseCommit and pushed the commits context — all from the
WithWaitingStatus worker, racing the UI thread. Read the marked base
hash before dispatching, and bounce the post-rebase reset and context
push through OnUIThread, still guarded by the success check so they
don't run on the conflict path.
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