have back button dismiss lightbox on web (#9874)

src/alf/atoms.ts

@@ -108,6 +108,22 @@ export const atoms = {
     animation: `zoomIn ${EXP_CURVE} 0.3s, fadeIn ${EXP_CURVE} 0.3s`,
   }),
 
+  /**
+   * Visually hidden but available to screen readers (web).
+   * Use for live regions or off-screen labels (e.g. "Image 1 of 3").
+   */
+  sr_only: web({
+    position: 'absolute',
+    width: 1,
+    height: 1,
+    padding: 0,
+    margin: -1,
+    overflow: 'hidden',
+    clip: 'rect(0,0,0,0)',
+    whiteSpace: 'nowrap',
+    borderWidth: 0,
+  }),
+
   /**
    * {@link Layout.SCROLLBAR_OFFSET}
    */

src/components/Post/Embed/VideoEmbed/VideoEmbedInner/VideoEmbedInnerWeb.tsx

@@ -72,19 +72,7 @@ export function VideoEmbedInnerWeb({
             loop={loop}
           />
           {embed.alt && (
-            <figcaption
-              id={figId}
-              style={{
-                position: 'absolute',
-                width: 1,
-                height: 1,
-                padding: 0,
-                margin: -1,
-                overflow: 'hidden',
-                clip: 'rect(0, 0, 0, 0)',
-                whiteSpace: 'nowrap',
-                borderWidth: 0,
-              }}>
+            <figcaption id={figId} style={a.sr_only}>
               {embed.alt}
             </figcaption>
           )}

src/view/com/lightbox/Lightbox.web.tsx

@@ -1,4 +1,4 @@
-import {useCallback, useEffect, useState} from 'react'
+import {useCallback, useEffect, useRef, useState} from 'react'
 import {Pressable, StyleSheet, View} from 'react-native'
 import {Image} from 'expo-image'
 import {msg} from '@lingui/macro'
@@ -69,7 +69,13 @@ function LightboxContainer({
       <Backdrop />
       <RemoveScrollBar />
       <FocusScope.FocusScope loop trapped asChild>
-        <div style={{position: 'absolute', inset: 0}}>{children}</div>
+        <div
+          role="dialog"
+          aria-modal="true"
+          aria-label={_(msg`Image viewer`)}
+          style={{position: 'absolute', inset: 0}}>
+          {children}
+        </div>
       </FocusScope.FocusScope>
     </Pressable>
   )
@@ -125,6 +131,32 @@ function LightboxGallery({
     return () => window.removeEventListener('keydown', onKeyDown)
   }, [onKeyDown])
 
+  // Push a history entry so the browser back button closes the lightbox
+  // instead of navigating away from the page.
+  const closedByPopStateRef = useRef(false)
+  useEffect(() => {
+    history.pushState({lightbox: true}, '')
+
+    const handlePopState = () => {
+      closedByPopStateRef.current = true
+      onClose()
+    }
+    window.addEventListener('popstate', handlePopState)
+
+    return () => {
+      window.removeEventListener('popstate', handlePopState)
+      // Only pop our entry if it's still the current one. If navigation
+      // already pushed a new entry on top, leave the orphaned entry —
+      // it shares the same URL so traversing through it is harmless.
+      if (
+        !closedByPopStateRef.current &&
+        (history.state as {lightbox?: boolean})?.lightbox
+      ) {
+        history.back()
+      }
+    }
+  }, [onClose])
+
   const delayedFadeInAnim = !reduceMotionEnabled && [
     a.fade_in,
     {animationDelay: '0.2s', animationFillMode: 'both'},
@@ -205,6 +237,11 @@ function LightboxGallery({
           </Pressable>
         </View>
       ) : null}
+      {imgs.length > 1 && (
+        <div aria-live="polite" aria-atomic="true" style={a.sr_only}>
+          <Text>{_(msg`Image ${index + 1} of ${imgs.length}`)}</Text>
+        </div>
+      )}
       <Button
         onPress={onClose}
         style={[