[buggedcom]

Summary

Hovering the history chart previously requested a full panel render and repeatedly rebuilt timeline labels. Cache date formatters and scale data, retain toolbar date references, and send hover timestamps directly to the toolbar.

 chart hover
-  reactive panel state → full panel render → recompute timeline labels
+  toolbar properties → toolbar update → reuse unchanged timeline labels

• Includes the formatter cache, timeline memoization, hover isolation, regression coverage, and rebuilt frontend bundle.
• Keeps comparison hover timestamps and hover clearing synchronized.

Type of change

• Bug fix
• Refactor / code quality
• Tests

Evidence

• Before: Five regression tests fail without timeline memoization, stable toolbar dates, and hover isolation.
  After: All pass; 215 scoped unit tests pass across 33 files.
• Before: The previously served HA bundle averaged 122 ms per step in a 20-step hover-update benchmark; settled idle produced six 80–95 ms long tasks in five seconds.
  After: The same benchmark averaged 16 ms per step. A final 40-step mouse sweep averaged 16.7 ms, handled all 40 hover events, and kept the toolbar marker synchronized. No long tasks were observed during the final five-second idle sample or sweep.
• Timings include two animation-frame waits per step and are not CPU execution times. The supplied URL had truncated page state; the restored entities and date range were held constant for the before/after comparison. With formatter/scale caching already active, restoring full panel updates remained within the same frame interval, so no separate timing gain is claimed for hover isolation.

Testing

• Scoped frontend unit tests: 215 passing
• Relevant Storybook stories: 3 passing
• Manually tested in Home Assistant using Chrome DevTools
• New GIVEN/WHEN/THEN regression tests verified against unfixed code
• pnpm build
• pnpm sb:build
• pnpm format
• pnpm lint:types

Checklist

• Commit messages follow Conventional Commits
• Frontend bundle rebuilt and committed
• Type checking passes
• Formatter and ESLint auto-fixes pass

Merge Danger

Door: two-way

Revert the commits and rebuild the bundle to restore the previous behavior.

Blast Radius: frontend

History panel hover markers, comparison markers, and timeline labels. Cache invalidation includes bounds, scale configuration, width, zoom, locale, and translation identity. No backend or persisted-state migration.