Address review findings: navigation edge cases, dropdown stacking, verse-0 dead code

[imnasnainaec]

Devin: https://app.devin.ai/review/sillsdev/interlinearizer-extension/pull/103

Follow-ups to the segment-view rework, addressing code-review findings. Two navigation edge-case bugs, one real-browser click-handling fix, removal of an unreachable code path, and documentation hardening around fragile patterns the review called out.

Navigation fixes

Continuous strip: external navigation interrupting an in-flight internal step. The strip tracks its pending phrase index in a ref so two rapid arrow clicks advance two groups before React re-renders. But when the parent imposed a new external focusedTokenRef while an internal step was still awaiting its echo, the in-flight marker suppressed the render-phase resync and the next step advanced from the stale index. The trigger is narrow but real: the focus-change effect early-returns when the new prop equals the currently displayed ref, so an external navigation landing on the displayed verse left the marker stuck indefinitely. The render-phase sync now detects an external override (the prop changed to something other than the in-flight internal ref), clears the marker, and resyncs the pending index. Covered by a regression test alongside the existing rapid-double-click test.

Internal-navigation markers now expire. When React batches rapid internal clicks into one update, the host echoes only the final reference, stranding the earlier verse's marker forever; a later external navigation to that verse would then be misclassified as internal and skip its recenter fade. Markers are now stamped (Map<verseKey, timestamp>) and expire after INTERNAL_NAV_TTL_MS (3 s — legitimate echoes round-trip in milliseconds, so nothing real can expire early). Both readers honor the TTL through a single shared freshness helper so they cannot drift: consumeInternalNav (which also evicts expired entries to bound the map) and the render-phase mid-reveal curtain guard (a pure read — no map mutation during render). Tests cover both the stranded-marker expiry and the mid-reveal re-engage with an expired marker naming the target verse.

View-options dropdown: gear toggle works in real browsers

The portaled click-catcher backdrop sat at z-20, while the toolbar (sticky + z-10) caps all its descendants at level 10 — so in a real browser the gear button could never receive a click while the dropdown was open, and the tested toggle-closed path only passed because jsdom ignores z-index for hit-testing. Raising the button cannot work (the toolbar's stacking context caps it), so the backdrop drops to z-5 instead: panel above toolbar above backdrop above page content. The backdrop is invisible, so this changes hit-testing only. One deliberate behavior change: while the dropdown is open, clicks on other toolbar controls now reach those controls directly instead of merely dismissing the dropdown.

Unreachable verse-0 handling removed

normalizeScrRef maps a verse-0 (chapter heading) selection to verse 1 before the reference ever reaches the display layer, so the chapter-heading active/snap branches in SegmentListView and useSegmentWindow.snapActiveToTop could never run in production — their tests only passed by injecting verseNum: 0 directly as a prop, bypassing the nav context. Those branches, the now-unconsumed data-chapter-start attribute, and their prop-injection tests are removed. Since the "scrRef is always normalized" assumption is now load-bearing, it is stated explicitly on Interlinearizer's scrRef prop.

Documentation hardening

The review flagged two effect dependency arrays as easy to break without understanding the design. Their comments now spell out the consequence: adding focusedTokenRef/tokenSegmentMap to the external-reseed effect would clobber the deliberately focused token on every focus move (they are guard-reads, not triggers), and the revert effect's correctness depends on isRevert staying derived directly from phraseMode.

Testing

Full Jest suite green (893 tests, 34 suites), ESLint/stylelint/typecheck clean.

This change is

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[imnasnainaec]

@alex-rawlings-yyc This pr is Claude Fable addressing Devin's remaining comments on #98 , but I haven't actually had a chance to review any of these changes myself. So as I break for lunch, you can decide whether you want to look at this yet.

[alex-rawlings-yyc]

I think these are worthy changes to make. Continue

@alex-rawlings-yyc reviewed 10 files and all commit messages, and made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on imnasnainaec).

[alex-rawlings-yyc]

@alex-rawlings-yyc reviewed 2 files and all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on imnasnainaec).