The body scroll-lock applied while the keyboard is up in main chat was an
iOS-specific workaround for visualViewport drift. On Android Chrome the same
mutation does the opposite of what we want — applying position:fixed to body
while the soft keyboard is opening causes Chrome to treat it as a focus-
target relayout and dismiss the keyboard instantly, making the main chat
composer unusable on Android.
useMobileScrollLock now early-returns on non-iOS user agents. Android Chrome
doesn't need it: with interactive-widget=resizes-content the layout viewport
shrinks with the keyboard, so there's no drift to compensate for.
Adds an Android-UA test case that asserts the lock is a no-op there.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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