A later streaming parse-cache change dropped the #MOBILESCROLL guard from
the live-stream render tick while leaving the helper and CSS in place.
On iOS PWA, Safari can paint a transient scrollTop=0 frame during streamed
DOM rebuilds, forcing the reader to the first/oldest message.
Re-add _fixMobileScrollJank() before streaming DOM work begins and pin it
with a regression test so the third layer of the mobile scroll fix cannot
be silently dropped again.
Brett Yelverton
2a40ca4240
