The desktop composer threw an uncaught "Composer is not available" at
startup and the input went unresponsive (#49903). assistant-ui's composer
mutators (setText/send/…) throw when the thread's composer core isn't bound
yet; the read path is null-safe but the writes are not. ChatBar pushes draft
text via aui.composer().setText() from mount-time effects (draft restore,
clearDraft, external inserts), and the v0.17.0 popout refactor (#49488)
widened the unbound window by moving the composer out of the contain wrapper
into a sibling of the thread — so the throw surfaced as an uncaught error
that wedged the input.
Wrap every composer mutation in a setComposerText helper that swallows the
unbound-core throw. The contentEditable DOM + draftRef already hold the text
and the draft-editor sync re-applies it once the core attaches, so the draft
is never lost — only the premature state push is skipped.
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