[Perf] Tooltip: every move re-renders the React chart, builds unused default content and rewrites unchanged attributes #187

Version: @tanstack/charts 1.1.0 (57774e1).

With a custom tooltip body (and with the motion() renderer for F), each pointer move that changes the tooltip does work no one sees: the React adapter re-renders the chart (A), core builds the default content the custom body replaces (E), and paint rewrites attributes that did not change (F). Related: #179 (only the default tooltip duration).

Paths are under packages/; links pin commit 57774e1.

A. Tooltip target state re-renders the chart (react-charts)

• useTooltipBody keeps the target in React.useState; onChange calls setTarget (tooltip.tsx:152-159).
• Each render rebuilds hostOptions, so the layout effect calls adapter.update (RendererChart.tsx:162-186). update refreshes DOM text metrics (renderer.ts:985-986).
• Repro (vitest + jsdom, 4 pointer moves): expected 3 to be +0 calls to adapter.update.
• Expected: a target change updates only the tooltip body. Proposed: keep the target in an external store read by a leaf component (useSyncExternalStore).
• Downstream measurement (bklit migration, Chrome, core Chart + external store): pointermove p95 2-6x lower.

B. A repeated hide() restarts the exit

Moved to #209 (a bug, together with C).

C. Finished tooltip animations stay attached

Moved to #209.

D. First show cannot be styled or delayed

Moved to #210 (a feature, together with the per-phase motion proposed under B).

E. Default content is always built

• paint always calls createChartTooltipContent before the custom body (tooltip.ts:117-124), and the result feeds the aria-label (:293).
• Repro: an items[].text formatter is called 1 time per paint with a custom body, expected 0.
• Expected: default content computed only when read. Proposed: resolve content lazily, or let a custom body supply the accessible label.
• Downstream measurement (bklit migration, Chrome CDP profile, 300 moves): about 6 ms per 100 moves; grows with series count.

F. Each paint rewrites the tooltip element (from this comment)

• On every move that repaints the tooltip, paint sets visibility: hidden and removes it again, and re-sets className, pointer-events, user-select and data-sticky, even when nothing changed (tooltip.ts:106-162). configureTooltipSemantics re-sets role, aria-live and aria-label.
• The spring glide writes style.translate from requestAnimationFrame every frame (motion.ts:419-428, motion.ts:484-497). The tween glide uses WAAPI instead.
• Measured (production build, sankey, MutationObserver on the tooltip, 60 moves): about 12 mutation records per move. About 4.7 are translate, 2.5 visibility, 3.75 the unchanged attributes, 1.8 content and 1 left/top.
• Proposed: compare before writing in paint. Run the spring glide as a sampled WAAPI translate, like the tween path.