fixup! ♻️(frontend) refactor screen share zoom pan with useMove and imperative DOM updates

src/frontend/src/features/rooms/livekit/hooks/useScreenShareZoom.ts

@@ -2,6 +2,7 @@ import { useCallback, useRef, useSyncExternalStore } from 'react'
 import { useMove } from 'react-aria'
 import type { MoveMoveEvent } from '@react-types/shared'
 import {
+  FULL_PICTURE_RATIO,
   MIN_ZOOM,
   PAN_STEP,
   WHEEL_ZOOM_SPEED,
@@ -14,6 +15,7 @@ import {
   getCursorFromZoomState,
   getCursorPercentsFromWheelEvent,
   getPanDeltaPercentsFromMove,
+  getPictureRatio,
   getWheelPanOffset,
   getZoomTransform,
 } from '../utils/screenShareZoom'
@@ -73,6 +75,22 @@ export const useScreenShareZoom = () => {
     el.style.transform = getZoomTransform(zoomRef.current, panRef.current)
   }, [])
 
+  // The video is letterboxed inside the surface by object-fit: contain, so the
+  // pan bounds depend on how much of the surface the picture actually covers.
+  // Read live rather than cached: both the tile and the shared resolution can
+  // change at any time.
+  const readPictureRatio = useCallback(() => {
+    const surface = surfaceElRef.current
+    const video = transformElRef.current?.querySelector('video')
+    if (!surface || !video) return FULL_PICTURE_RATIO
+    return getPictureRatio(
+      surface.clientWidth,
+      surface.clientHeight,
+      video.videoWidth,
+      video.videoHeight
+    )
+  }, [])
+
   const applyCursor = useCallback(() => {
     const el = surfaceElRef.current
     if (!el) return
@@ -85,21 +103,23 @@ export const useScreenShareZoom = () => {
   const zoomIn = useCallback(() => {
     const next = clampZoom(zoomRef.current + ZOOM_STEP)
     zoomRef.current = next
-    panRef.current = clampPan(panRef.current, next)
+    panRef.current = clampPan(panRef.current, next, readPictureRatio())
     applyTransform()
     applyCursor()
     flush()
-  }, [applyTransform, applyCursor, flush])
+  }, [applyTransform, applyCursor, flush, readPictureRatio])
 
   const zoomOut = useCallback(() => {
     const next = clampZoom(zoomRef.current - ZOOM_STEP)
     zoomRef.current = next
     panRef.current =
-      next <= MIN_ZOOM ? { x: 0, y: 0 } : clampPan(panRef.current, next)
+      next <= MIN_ZOOM
+        ? { x: 0, y: 0 }
+        : clampPan(panRef.current, next, readPictureRatio())
     applyTransform()
     applyCursor()
     flush()
-  }, [applyTransform, applyCursor, flush])
+  }, [applyTransform, applyCursor, flush, readPictureRatio])
 
   const resetZoom = useCallback(() => {
     zoomRef.current = MIN_ZOOM
@@ -136,6 +156,7 @@ export const useScreenShareZoom = () => {
           nextZoom: next,
           cursorXPercent,
           cursorYPercent,
+          ratio: readPictureRatio(),
         })
       }
 
@@ -143,7 +164,7 @@ export const useScreenShareZoom = () => {
       applyCursor()
       flush()
     },
-    [applyTransform, applyCursor, flush]
+    [applyTransform, applyCursor, flush, readPictureRatio]
   )
 
   // useMove handles mouse drag + touch pan. Keyboard arrows are not handled
@@ -173,7 +194,8 @@ export const useScreenShareZoom = () => {
           x: panRef.current.x + deltaXPercent,
           y: panRef.current.y + deltaYPercent,
         },
-        zoomRef.current
+        zoomRef.current,
+        readPictureRatio()
       )
 
       applyTransform()
@@ -195,12 +217,13 @@ export const useScreenShareZoom = () => {
     (dx: number, dy: number) => {
       panRef.current = clampPan(
         { x: panRef.current.x + dx, y: panRef.current.y + dy },
-        zoomRef.current
+        zoomRef.current,
+        readPictureRatio()
       )
       applyTransform()
       flush()
     },
-    [applyTransform, flush]
+    [applyTransform, flush, readPictureRatio]
   )
 
   // Attached to the tile container (not the zoom surface) where keyboard

src/frontend/src/features/rooms/livekit/utils/screenShareZoom.ts

@@ -10,6 +10,14 @@ export interface PanOffset {
   y: number
 }
 
+// Fraction of the surface each axis of the picture covers, in [0, 1].
+export interface PictureRatio {
+  x: number
+  y: number
+}
+
+export const FULL_PICTURE_RATIO: PictureRatio = { x: 1, y: 1 }
+
 export interface ZoomSnapshot {
   zoomLevel: number
   zoomPercentage: number
@@ -24,15 +32,41 @@ export const clampZoom = (value: number) => {
   return Math.max(MIN_ZOOM, Math.min(MAX_ZOOM, value))
 }
 
-// Restrict pan so the picture always covers the view.
-export const clampPan = (pan: PanOffset, zoom: number): PanOffset => {
-  const maxPan = ((zoom - 1) / zoom) * PAN_CLAMP_HALF
+// Restrict pan so the picture always covers the view. Pan is a % of the
+// surface, in which object-fit: contain letterboxes the picture: its half
+// extent is `ratio * 50` against a view half extent of 50, and scaling by
+// `zoom` must keep `zoom * (ratio * 50 - |pan|) >= 50`. An axis whose picture
+// is still smaller than the view is pinned to 0, keeping the bars symmetric.
+export const clampPan = (
+  pan: PanOffset,
+  zoom: number,
+  ratio: PictureRatio
+): PanOffset => {
+  const maxPanX = Math.max(0, (ratio.x - 1 / zoom) * PAN_CLAMP_HALF)
+  const maxPanY = Math.max(0, (ratio.y - 1 / zoom) * PAN_CLAMP_HALF)
   return {
-    x: Math.max(-maxPan, Math.min(maxPan, pan.x)),
-    y: Math.max(-maxPan, Math.min(maxPan, pan.y)),
+    x: Math.max(-maxPanX, Math.min(maxPanX, pan.x)),
+    y: Math.max(-maxPanY, Math.min(maxPanY, pan.y)),
   }
 }
 
+// Per-axis fraction of the surface covered by an object-fit: contain picture.
+export const getPictureRatio = (
+  surfaceWidth: number,
+  surfaceHeight: number,
+  videoWidth: number,
+  videoHeight: number
+): PictureRatio => {
+  if (!surfaceWidth || !surfaceHeight || !videoWidth || !videoHeight) {
+    return FULL_PICTURE_RATIO
+  }
+  const surfaceRatio = surfaceWidth / surfaceHeight
+  const videoRatio = videoWidth / videoHeight
+  return surfaceRatio > videoRatio
+    ? { x: videoRatio / surfaceRatio, y: 1 }
+    : { x: 1, y: surfaceRatio / videoRatio }
+}
+
 export const buildZoomSnapshot = (
   zoom: number,
   pan: PanOffset,
@@ -68,12 +102,14 @@ export const getWheelPanOffset = ({
   nextZoom,
   cursorXPercent,
   cursorYPercent,
+  ratio,
 }: {
   pan: PanOffset
   prevZoom: number
   nextZoom: number
   cursorXPercent: number
   cursorYPercent: number
+  ratio: PictureRatio
 }): PanOffset => {
   const panShift = 1 / nextZoom - 1 / prevZoom
   return clampPan(
@@ -81,7 +117,8 @@ export const getWheelPanOffset = ({
       x: pan.x + cursorXPercent * panShift,
       y: pan.y + cursorYPercent * panShift,
     },
-    nextZoom
+    nextZoom,
+    ratio
   )
 }