⚡️(frontend) optimize re-renders of the CarouselLayout

Apply the same pattern as previous layouts: push state and useSize
subscriptions down into child components.

The CarouselLayout now only re-renders when maxVisibles or
orientation actually change, instead of on every size update.

src/frontend/src/features/layout/components/CarouselLayout.tsx

@@ -3,6 +3,7 @@ import { getScrollBarWidth } from '@livekit/components-core'
 import * as React from 'react'
 import { TrackLoop, useVisualStableUpdate } from '@livekit/components-react'
 import { useSize } from '@/features/rooms/livekit/hooks/useResizeObserver'
+import { useCallback, useEffect, useLayoutEffect } from 'react'
 
 const MIN_HEIGHT = 130
 const MIN_WIDTH = 140
@@ -10,6 +11,72 @@ const MIN_VISIBLE_TILES = 1
 const ASPECT_RATIO = 16 / 10
 const ASPECT_RATIO_INVERT = (1 - ASPECT_RATIO) * -1
 
+type CarouselOrientation = 'vertical' | 'horizontal'
+
+interface CarouselLayoutState {
+  orientation: CarouselOrientation
+  maxVisibleTiles: number
+}
+
+interface CarouselLayoutObserverProps {
+  asideEl: React.RefObject<HTMLDivElement>
+  orientation?: CarouselOrientation
+  onLayoutChange: (layout: CarouselLayoutState) => void
+}
+
+const CarouselLayoutObserver = ({
+  orientation,
+  asideEl,
+  onLayoutChange,
+}: CarouselLayoutObserverProps) => {
+  const { width, height } = useSize(asideEl)
+
+  // Hysteresis memory: avoids flapping between N and N+1 tiles when the
+  // container size hovers around a breakpoint. A ref (not state) because
+  // updating it must not trigger a re-render.
+  const prevTilesRef = React.useRef(0)
+
+  const carouselOrientation: CarouselOrientation =
+    orientation ?? (height >= width ? 'vertical' : 'horizontal')
+
+  const tileSpan =
+    carouselOrientation === 'vertical'
+      ? Math.max(width * ASPECT_RATIO_INVERT, MIN_HEIGHT)
+      : Math.max(height * ASPECT_RATIO, MIN_WIDTH)
+
+  const scrollBarWidth = getScrollBarWidth()
+
+  const availableSpan =
+    (carouselOrientation === 'vertical' ? height : width) - scrollBarWidth
+
+  const tilesThatFit = Math.max(availableSpan / tileSpan, MIN_VISIBLE_TILES)
+
+  let maxVisibleTiles: number
+  if (Math.abs(tilesThatFit - prevTilesRef.current) < 0.5) {
+    // Within the dead zone: keep the previous count.
+    maxVisibleTiles = Math.round(prevTilesRef.current)
+  } else {
+    maxVisibleTiles = Math.round(tilesThatFit)
+    prevTilesRef.current = tilesThatFit
+  }
+
+  // Apply cosmetic layout output straight to the DOM.
+  useLayoutEffect(() => {
+    const el = asideEl.current
+    if (!el) return
+    el.dataset.lkOrientation = carouselOrientation
+    el.style.setProperty('--lk-max-visible-tiles', maxVisibleTiles.toString())
+  }, [asideEl, carouselOrientation, maxVisibleTiles])
+
+  // Report upward only what the parent actually needs for
+  // `useVisualStableUpdate` (and only when it changes — see parent handler).
+  useEffect(() => {
+    onLayoutChange({ orientation: carouselOrientation, maxVisibleTiles })
+  }, [carouselOrientation, maxVisibleTiles, onLayoutChange])
+
+  return null
+}
+
 /** @public */
 export interface CarouselLayoutProps extends React.HTMLAttributes<HTMLMediaElement> {
   tracks: TrackReferenceOrPlaceholder[]
@@ -40,52 +107,42 @@ export function CarouselLayout({
   ...props
 }: CarouselLayoutProps) {
   const asideEl = React.useRef<HTMLDivElement>(null)
-  const [prevTiles, setPrevTiles] = React.useState(0)
-  const { width, height } = useSize(asideEl)
-  const carouselOrientation = orientation
-    ? orientation
-    : height >= width
-      ? 'vertical'
-      : 'horizontal'
 
-  const tileSpan =
-    carouselOrientation === 'vertical'
-      ? Math.max(width * ASPECT_RATIO_INVERT, MIN_HEIGHT)
-      : Math.max(height * ASPECT_RATIO, MIN_WIDTH)
-  const scrollBarWidth = getScrollBarWidth()
+  const [layout, setLayout] = React.useState<CarouselLayoutState>({
+    orientation: orientation ?? 'vertical',
+    maxVisibleTiles: MIN_VISIBLE_TILES,
+  })
 
-  const tilesThatFit =
-    carouselOrientation === 'vertical'
-      ? Math.max((height - scrollBarWidth) / tileSpan, MIN_VISIBLE_TILES)
-      : Math.max((width - scrollBarWidth) / tileSpan, MIN_VISIBLE_TILES)
+  // Stable callback + identity check: the parent only re-renders when the
+  // derived layout genuinely changed, not on every resize tick.
+  const handleLayoutChange = useCallback((next: CarouselLayoutState) => {
+    setLayout((prev) =>
+      prev.orientation === next.orientation &&
+      prev.maxVisibleTiles === next.maxVisibleTiles
+        ? prev
+        : next
+    )
+  }, [])
 
-  let maxVisibleTiles = Math.round(tilesThatFit)
-  if (Math.abs(tilesThatFit - prevTiles) < 0.5) {
-    maxVisibleTiles = Math.round(prevTiles)
-  } else if (prevTiles !== tilesThatFit) {
-    setPrevTiles(tilesThatFit)
-  }
-
-  const sortedTiles = useVisualStableUpdate(tracks, maxVisibleTiles)
-
-  React.useLayoutEffect(() => {
-    if (asideEl.current) {
-      asideEl.current.dataset.lkOrientation = carouselOrientation
-      asideEl.current.style.setProperty(
-        '--lk-max-visible-tiles',
-        maxVisibleTiles.toString()
-      )
-    }
-  }, [maxVisibleTiles, carouselOrientation])
+  const sortedTiles = useVisualStableUpdate(tracks, layout.maxVisibleTiles)
 
   return (
-    <aside
-      key={carouselOrientation}
-      className="lk-carousel"
-      ref={asideEl}
-      {...props}
-    >
-      <TrackLoop tracks={sortedTiles}>{props.children}</TrackLoop>
-    </aside>
+    <>
+      <CarouselLayoutObserver
+        asideEl={asideEl}
+        orientation={orientation}
+        onLayoutChange={handleLayoutChange}
+      />
+      {/* `key` intentionally remounts the container when orientation flips, */}
+      {/* which resets scroll position and re-runs the observer measurement. */}
+      <aside
+        key={layout.orientation}
+        className="lk-carousel"
+        ref={asideEl}
+        {...props}
+      >
+        <TrackLoop tracks={sortedTiles}>{props.children}</TrackLoop>
+      </aside>
+    </>
   )
 }