Fixed on canary by dd148271 — gateway-acknowledged message creates now advance the read watermark in the same state emission that appends the message, so there is no frame where the NEW divider can render. Covered by regression tests in chat_view_model_unread_test.dart and message_list_scroll_test.dart. Will be in the next beta; please reopen if it still reproduces there.
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Comment by @M0n7y5