Why: reproduced with headless Chromium - clicking an artifact row with no
conversation selected closed the workspace overlay and nothing else appeared.
requestPreview stores tabs under currentConvId ?? '_' while the render side
required a non-null currentConvId, so the preview pane never mounted in the
empty state. The empty state was itself a trap: currentConvId was not
persisted, so every reload landed there.
Evidence: vitest 85/85 (new preview-empty-state tests for the key contract
and orphan-tab merge); browser replay on the fixed build shows the preview
pane mounting with rendered docx content from the empty state and the
conversation restoring after reload (screenshots in the Dev task card).
Impact: previewTabsKey() unifies the read/write key; entering a conversation
adopts orphan '_' tabs; the last conversation id persists and restores once
on boot; deliverable cards toast when the artifact is gone instead of
silently doing nothing.
Refs: Dev task feature/empty-state-preview-fix; diagnosis session 2026-10-10.
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