The mobile chat keyboard collapsed the instant it opened on iOS because
the keyboard scroll-lock pinned body to position:fixed a beat after the
composer was already focused — pinning an ancestor to position:fixed
after focus blurs the input on iOS Safari (no visible jump, since the
dashboard base layout is already at scrollY 0).
Add useMobileKeyboardViewportLock: an overflow-only viewport lock
(overflow:hidden on html/body + scrollTo(0,0)) that does NOT touch
position, mirroring the working Quick Chat panel. App-level and ChatView
keyboard pins use it now; modals keep the position:fixed lock unchanged.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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