While a fold's anchor held, the LegendList patch skipped end maintenance
on every pass, but data-change passes don't apply the anchor. A row
added within that window, as a streaming reply does, was left below the
viewport of a thread that was following its end.

The patch now records whether a pass is a data change and skips end
maintenance only after the size-change passes the anchor places.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>