Root cause: .messages had overflow-anchor:none unconditionally. On mobile
touch devices (iOS Safari, Android Chrome), the compositor can paint a frame
with scrollTop=0 between innerHTML='' and snapshot restore, scroll-janking
the user to the top of the transcript.
Three-layer fix:
1. CSS (style.css): Use overflow-anchor:auto on touch devices (mobile),
keep overflow-anchor:none on desktop (mouse-driven) via
@media (hover:hover) and (pointer:fine) — preserves the existing
behavior where tool/card inserts don't yank a reading user.
2. JS guard (ui.js): Add window._fixMobileScrollJank() that temporarily
sets overflowAnchor='auto' on #messages before innerHTML='' and
clears it one rAF later — prevents the compositor frame with scrollTop=0.
3. Streaming protection (messages.js): Call _fixMobileScrollJank() at the
top of each _doRender() tick so streaming DOM growth doesn't jank the
viewport position.
Tests:
- Updated test_issue1360_streaming_scroll_hardening.py: split the
overflow-anchor test into desktop (none) and mobile (auto) variants.
- Added test_mobile_scroll_jank.html: interactive visual test that
simulates the scroll jank on a mobile-sized viewport.
