startTurnWithImages clears the previous turn's tool-call overlay and
pins scrollOffset to 0. Without also resetting prevChatLen/prevChatCols,
the auto-follow guard on the next render sees a synthetic negative
delta equal to the number of overlay rows that were cleared, and
nudges scrollOffset by that amount. On terminals that mirror zot's
chat-pane scroll into their native scrollbar this is visible as a
viewport jump the instant the user presses enter on a follow-up
prompt.
Zero them out in the same locked block so the guard short-circuits
on the very next render, the same way it already does on column
resize. The legitimate "user scrolled up while content streams in"
case is unaffected because prevChatLen is repopulated on that first
post-submit render.