Fix tab activation regression in search highlighting

[cderv]

Important

This PR targets the feature/robust-search-highlighting branch (#14080), not main. It is intended to be merged into #14080 before that PR merges into main.

Context

PR #14080 replaced activateTabsWithMatches() (from #14053) with openAllTabsetsContainingEl(), which is a much cleaner approach — 9 lines vs 65 — but dropped two behaviors that the playwright tests in html-search-tabsets.spec.ts verify.

What regressed

1. "Keep active tab" logic — when multiple panes in the same tabset contain a match, the old code kept the already-active tab. The new code unconditionally activated each pane, so the last match processed won.

2. Pageshow timing — tab activation ran during DOMContentLoaded, but tabsets.js restores tab state from localStorage on pageshow (which fires after). So localStorage overwrote search activation.

Fix

Adapt openAllTabsetsContainingEl (not replace it) with minimal changes:

• Before activating a pane, check if the same tabset's active pane already has a <mark> — skip if so
• Move tab activation from inside highlight() to a pageshow handler, so it runs after tabsets.js
• Add scrollToFirstVisibleMatch that skips marks inside inactive tab panes
• Fix for (leaf of leafNodes) → for (const leaf of leafNodes) (implicit global)

The adapted function stays at 9 lines (vs the 65-line activateTabsWithMatches it replaced). highlight() is simplified to marking-only.

Test

All 6 tests × 3 browsers pass locally:

Running 18 tests using 11 workers
  18 passed (16.3s)

[posit-snyk-bot]

✅ Snyk checks have passed. No issues have been found so far.

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