A turn that ends in an error (e.g. an out-of-funds state) was being
re-rendered in unrelated threads. On a warm thread switch the on-screen
`$messages` still belongs to the previously viewed thread, and
`flushPendingViewState` fed it into `preserveLocalAssistantErrors`, which
grafted the prior thread's failed turn onto the newly opened one. Because
the polluted view then became the next switch's baseline, the error
cascaded into every thread the user visited.
Only carry local errors across a view flush when the on-screen baseline is
the same session being flushed; the cached state we publish already retains
that session's own errors. Also surface the turn error as a global toast
even when the failing turn ran in a background thread, since the error
blocks all subsequent interactions until the user acts.
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