perf: cut redundant DOM work in the section-map poller and difficulty updates

[carochacs]

Summary

Two verified performance fixes, no behavior change:

1. _smUpdate() did redundant DOM work every 200ms regardless of change — it called document.getElementById('sm-marker') and _smBar.querySelectorAll('.sm-block') fresh on every 5Hz poller tick, and force-wrote style.opacity on every block on every tick even when the active section hadn't changed since the last tick. _smRender() now caches the marker element and block array once, right after building the bar (invalidated on _smRemove()); _smUpdate() reads from the cache and only flips opacity on the previous/new active block when the active index actually changes.
2. difficulty:sections-updated handler always called a full _smRender() — every difficulty-data push from dynamic-difficulty tore down and rebuilt every section block's entire DOM purely to redraw a handful of small "glass fill" percentage bars. Added _smUpdateDifficultyFills(), which updates each glass element's fill height/title in place, falling back to a full _smRender() only if the cached blocks don't match the current section count.

Both changes preserve every plugin-spec rule from CLAUDE.md: idempotent guard untouched, screen:changed mount/unmount untouched, seeking still routed through _smSeek, section/difficulty data still read only via window.highway/the event bus, no MutationObserver/DOM polling introduced.

Bumped plugin.json version 1.2.3 → 1.2.4 (patch) per the CLAUDE.md versioning rule.

Testing

• node -c screen.js — clean
• tests/screen.test.js — all 16 existing tests pass unchanged
• Manual DOM-simulation scripts (not committed) confirmed querySelectorAll/getElementById now run once per render rather than once per tick, and difficulty updates no longer trigger full innerHTML rebuilds

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[pullfrog]

ℹ️ Solid perf work — one gap on test coverage.

Reviewed changes

Two targeted performance optimizations to screen.js, no behavior change:

• Poller DOM ref caching: _smRender() caches the marker element and block array once after building the bar; _smUpdate() reads from the cache and only touches the two blocks whose opacity actually changed, instead of querySelectorAll + forEach on every 200ms tick.
• Incremental difficulty fills: _smUpdateDifficultyFills() updates glass fill height/title in place on difficulty:sections-updated events, avoiding a full innerHTML rebuild of every section block. Falls back to _smRender() when cached blocks are stale.

Both changes preserve the idempotent guard, screen:changed mount/unmount, _smSeek seeking path, and window.highway data reads. Version bump 1.2.3 → 1.2.4 per spec.

  ^{｜ Fix all ➔ ｜ Fix 👍s ➔ ｜ View workflow run ｜ Using Big Pickle (free) ｜ 𝕏}

[pullfrog]

✅ No new issues found.

Reviewed changes

Test coverage commit (7bedc3c) directly addressing the prior review's flagged gap:

• Opacity bake contract — _smRender test asserts exactly 3 opacity:0.5 matches in the rendered HTML (one per section), catching any removal of the baked-in default that the cached highlight depends on.
• Cached highlight path — Injected mock blockEls with specific values; _smUpdate only touches the two blocks whose active state flipped ('1' / '0.5'), untouched blocks stay undefined.
• _smUpdateDifficultyFills branches — Three tests cover the in-place update (glass identity preserved, fill height/title updated, no insertAdjacentHTML), size-change rebuild (old glass removed, new one inserted), and stale removal (glass removed when difficulty data is absent).

Assertions are specific (identity checks, exact values, counts) rather than truthy, so each test would fail if the optimization it targets was broken.

  ^{｜ View workflow run ｜ Using Big Pickle (free) ｜ 𝕏}