A turn that streamed only prose (a long plain-text answer, or a degeneration
burst that floods the body with repeated tokens) still projected an anchor
activity scene whose activity_rows were all prose/terminal — zero tool/thinking
rows. The settle path promoted it to a collapsed worklog anyway (the gate only
checked activity_rows.length), hiding the whole answer and shrinking the
transcript by the full streamed height at STREAM_DONE, so the browser clamped a
bottom-pinned viewport back to the top (the 'jump back' report).
Add a worklog-worthiness predicate at both gates: the generation gate
(_anchorSceneHasWorklogWorthyRows in messages.js, decides whether to attach a
scene at all) and the render gate (_anchorSceneSceneHasWorklogWorthyRows in
ui.js, defense-in-depth for already-persisted all-prose scenes). A scene is
worklog-worthy only if it has >=1 tool/thinking row or a compression lifecycle
card; pure prose is not, so the turn renders as normal visible prose and the
viewport stays pinned at the bottom.
Behavioral + structural tests in tests/test_pure_prose_turn_not_worklog.py.
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