A floor switch replaces the whole stage, so the card's pointer-hover
MutationObserver receives hundreds of records whose targets are the same
few containers. Each record re-ran `matches` and a `.devlayer` subtree
`querySelector` on its target, and kept doing so after the device layer
had already been found. The batch logic moves to `deviceLayerMutated` in
device-hit-owner.ts: a node is checked at most once per batch, the first
hit ends the checks, and every added node still goes through
`_syncPointerHoverSubtree` in record order. The card shrinks by 12 lines.
The View stair layer read the card's `_model` getter once more for every
navigable stair; the getter rebuilds the config fingerprint on each read.
`renderLayer` now reads it once.
`languageRenderGate` wrote `lang` on the host on every render. It now
writes it only when the value differs (language switch, English fallback,
a foreign value); an unchanged value is left alone.
No behaviour changes: DOM, tooltips and pixels are the same. Unit tests
count subtree queries per node, `_model` reads per render and `lang`
writes; one mutant per change restores the old behaviour.
Issue: #694
User-Visible: no
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018qZfe7YS4rqEMKoVeS3GKd
