fix: canonicalize the frozen-row boundary across all comparison sites

[ghiscoding]

verified by Copilot using GPT-5.3-Codex

Port bug fix from 6pac/SlickGrid PR 6pac/SlickGrid#1268 into slickgrid-universal

⚠️ Merge order: this PR assumes 6pac/SlickGrid#1266 and 6pac/SlickGrid#1267 are merged first. It has no textual conflicts with either, but it was authored and validated against a master that includes them — please land those two before this one.

The bug (Q3 in discussion 6pac/SlickGrid#1247)

The frozen-row boundary was compared differently at six sites, contradicting each other and the render split (rows >= actualFrozenRow render in the bottom canvas):

Site Drift Observable
appendRowHtml frozen class row <= frozenRow (a count compare) bottom mode: the first rows classed .frozen, the actually-pinned rows never; top mode: off-by-one
cleanupRows / cleanUpCells <= actualFrozenRow first scrollable row never evicted/cleaned in top mode
_getContainerElement (getCanvasNode/getViewportNode) >= actualFrozenRow + 1 in top mode returns the TOP pane for a row whose DOM lives in the BOTTOM canvas — overlays/menus positioned via the public API mis-target exactly the boundary row
scrollRowIntoView actualFrozenRow - 1 boundaries bottom mode: refuses to scroll the LAST scrollable row (navigateToPos, one screen away, already compares correctly)

The fix

Two predicates matching the render split — isBottomBandRow (row >= actualFrozenRow) and isFrozenRowIdx (frozenBottom ? row >= actualFrozenRow : row < actualFrozenRow) — with every site routed through them. Net −5 lines.

Downstream check (executed)

slickgrid-universal maintains its own fork of this grid (verified by clone + grep — it does not consume this file at runtime), so this cannot break it. Its fork carries the same isBottomSide off-by-one and may want to port the fix.

Test

cypress/e2e/quirk-frozen-row-boundary.cy.ts — self-hosting (harness via cy.intercept; no example-page dependency), one check per drifted site plus a frozen-row-stays-cached control. Pre-fix it fails on every drifted site with the predicted signatures (frozen class [0,1,2,3] in both modes; getCanvasNode returning the top canvas while the row's DOM is in the bottom; scrollRowIntoView(996) leaving viewport.top=0; boundary row never evicted); post-fix all pass. Frozen-family suites including the merged #1255/#1258 quirk specs ran 27/27.

⚠️ Temporary example page

examples/example-quirk-frozen-row-boundary.html (tinted .frozen rows + pane-lookup/scroll readouts) is intended to be deleted before merge — the cypress test is fully independent of it.

(Quirks-triage remaining-items wave: see discussion 6pac/SlickGrid#1247; siblings 6pac/SlickGrid#1266, 6pac/SlickGrid#1267.)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.0%. Comparing base (9813849) to head (7731d5f).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2706   +/-   ##
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  Coverage   100.0%   100.0%           
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  Files         200      200           
  Lines       25503    25496    -7     
  Branches     9020     9010   -10     
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- Hits        25503    25496    -7     

Flag	Coverage Δ
angular	100.0% <ø> (ø)
universal	100.0% <100.0%> (ø)

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commit: 7731d5f