Auto-scroll now yields when the reader scrolls away from the bottom, so a
fast-streaming reply no longer fights someone reading the start. Stickiness is
a flag driven by a passive scroll listener (not a per-flush distance check,
which breaks when one flush adds more than the threshold of content); scrolling
back within the band re-arms it.
Centralised in chat-session.js (_scrollToBottom(force) + _forceScrollToBottom
+ _messagesContainer hook), removing the duplicated overrides in copilot.js /
chat-page.js. Force-scroll is used where the latest must be shown: history
load, chat reopen, approval/clarification prompts, and message send.
A sticky 'jump to latest' affordance appears only while scrolled up.
dguiducci