Codex found the guard had a first-event gap: programmatic seeds of _lastScrollTop
(_setMessageScrollToBottom, scrollToBottom, snapshot restore paths) didn't seed
_lastMessageClientHeight, so the FIRST native toolbar-collapse scroll saw a null
prior height -> grew=false -> the false-unpin still fired. Now every programmatic
_lastScrollTop=el.scrollTop is paired with _lastMessageClientHeight=el.clientHeight.