Finalized assistant messages rendered the thinking/tools trail inside
MessageLine with marginBottom=1 before the response body — giving a
clean blank line above the text. The streaming path rendered the
progress ToolTrail and the streaming MessageLine as two separate
siblings with no margin between, so the in-progress response butted
right up against the thinking panel. That's the "newline appears
after it's done" jank.
Wrap the streaming MessageLine in a Box with marginTop=1 whenever the
progress area is visible above it. Same spacing as the finalized
version, continuous through the handoff.
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