Spreadsheet row identity, header rows under a title, and a Mermaid pair that opens as a picture

[mindaugaskasp]

Closes the Spreadsheet track's two now items on docs/roadmap.md, adds the cross-panel row hover, and fixes a Mermaid pair that came back as text.

Row identity — key columns

utils/matchRowsByKey.js pairs rows by a composite key across the whole sheet instead of by whole-row LCS, so the same export sorted differently reads as the figures that moved rather than a rewrite. Duplicate keys pair in order of occurrence and are counted, never hidden — a cap that hides is worse than a cap. Chosen per sheet in the new Match-rows popover; Auto keeps today's LCS exactly, because changing the default matcher for every existing diff has no failing case behind it.

Display order follows the RIGHT file, with a removed row emitted just before the first surviving left row that followed it, so it keeps its context.

Header row offset

headerPairing scans the first ten rows for a usable header instead of reading rows[0] and dropping silently to positional pairing — the exact failure alignColumns exists to prevent. Each side scans independently, since a title row added on one side only is the common case, and the status band names the row it used when it was not the first.

Row hover across both grids

The hovered row is marked in both panels — they are two <table>s, so :hover in one cannot reach the row aligned with it in the other. An outline, not a veil: every row and cell state here is already a background, and a wider border would move the rows the virtualization measures. 2px inset, because 1px read as a different border on beacon and contrast, whose hard --border is a contract.

A restored Mermaid pair opens as a picture

Every load path already defaulted to the diagram — file dialog, drop, diffbro compare, relaunch — all verified against a real launch. restore() was the gap: it replayed payload.semanticView === true, so a snapshot that recorded nothing came back as text. The view is now a tri-state on the way in; an explicitly recorded view is kept either way.

Bugs found and fixed

• keyColumnsOf accepted an out-of-range column (undefined !== null is true) and threw.
• The Match-rows panel was unclickable: .content carries isolation: isolate, so the body-teleported backdrop outranked the whole diff subtree.
• Three of seven new header tests passed before the fix — positional pairing coincided with the right answer. Rewritten so the two pairings disagree.
• toolbar-view-menu.spec.mjs still selected .count after the chip moved to ui.css as .btn-count.
• The first restore() attempt made a new tab inherit the previous tab's toggle; tabsStore.test.js caught it.

Verification

• npm run check green: 3088 tests, coverage 95.34 / 88.47 / 95.76 / 96.33 over the 93 / 86 / 92 / 95 floors, theme depth ok on 20 themes, i18n and raw-text clean.
• Full e2e in the container, plus targeted host runs of every spec touching the changed contracts.
• The hover mark read off real frames on light, matrix, beacon, contrast, sepia, nyan and volcano.
• useSpreadsheetDiff hit its 73-line ratchet and split into useToleranceChoice + useKeyColumns; entry retightened 73 → 69, never raised.

Spec: specs/2026-08-09-spreadsheet-row-identity/plan.md.

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