"Show as walkthrough" in Safari's engine: before and after

On the Mac app, which renders with Safari's engine, Obeya's chat stopped following new messages once a walkthrough was requested. The walkthrough then played below the visible part of the chat. Both shots below were taken in WebKit: an older answer was clicked, and the shot was taken about 90 seconds later, with the walkthrough playing.

Before: the chat ends at the answer's first lines — **Before**The chat stops at the start of the answer. The playing walkthrough is 514 pixels further down, out of sight.

After: the walkthrough player is in view — **After**The chat follows to its end: the answer and the playing walkthrough are in view (0 pixels below).

Cause: as a message is replaced (Obeya's "reading" line by its reply), WebKit moves the chat up by a few dozen pixels on its own. The chat read that move as the owner scrolling up and stopped following. It now reads only a wheel, touch or key in the chat as the owner scrolling. Chrome doesn't move the chat this way, so the fault never showed there.