Previous textarea-only touchmove listener didn't catch swipes that
started elsewhere (header, composer padding) — those still panned the
iOS visualViewport over the locked document, sliding the composer up.
Replaces it with a document-level non-passive touchmove listener that
fires while keyboardOpen on mobile and preventDefaults all gestures
EXCEPT when the target is inside .chat-messages (the one container
where pan-y should still work). Stops both header-swipe and composer-
swipe from panning the page, while leaving messages-list scroll
intact.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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