ensure alt text is scrollable (#10671)

Co-authored-by: Eric Bailey <git@esb.lol>

src/components/Lightbox/Lightbox.web.tsx

@@ -1,5 +1,5 @@
 import {useCallback, useEffect, useRef, useState} from 'react'
-import {Pressable, StyleSheet, View} from 'react-native'
+import {Pressable, ScrollView, StyleSheet, View} from 'react-native'
 import {Image} from 'expo-image'
 import {Trans, useLingui} from '@lingui/react/macro'
 import {FocusGuards, FocusScope} from 'radix-ui/internal'
@@ -226,17 +226,21 @@ function LightboxGallery({
         )}
       </View>
       {img.alt ? (
-        <View
+        <ScrollView
+          // Cap the overlay height so long alt text scrolls within the overlay
+          // instead of growing past the top of the screen and pushing the image
+          // out of view. Only scrollable once expanded.
           style={[
-            a.px_4xl,
+            styles.altScroll,
-            a.py_2xl,
             {
               backgroundColor: 'rgba(0, 0, 0, 0.5)',
               // @ts-expect-error web only
               backdropFilter: 'blur(16px)',
             },
             delayedFadeInAnim,
-          ]}>
+          ]}
+          scrollEnabled={isAltExpanded}
+          contentContainerStyle={[a.px_4xl, a.py_2xl]}>
           <Pressable
             accessibilityLabel={l`Expand alt text`}
             accessibilityHint={l`If alt text is long, toggles alt text expanded state`}
@@ -250,7 +254,7 @@ function LightboxGallery({
               {img.alt}
             </Text>
           </Pressable>
-        </View>
+        </ScrollView>
       ) : null}
       {imgs.length > 1 && (
         <div aria-live="polite" aria-atomic="true" style={a.sr_only}>
@@ -449,6 +453,14 @@ const styles = StyleSheet.create({
     padding: 16,
     boxSizing: 'border-box',
   },
+  altScroll: {
+    // Size to content like the View it replaced, rather than filling the
+    // column via ScrollView's default flexGrow.
+    flexGrow: 0,
+    flexShrink: 0,
+    // @ts-ignore web-only -sfn
+    maxHeight: '50vh',
+  },
   menuBtn: {
     top: 20,
     left: 20,

src/components/Lightbox/chrome/Footer.tsx

@@ -1,6 +1,9 @@
 import {useRef} from 'react'
 import {LayoutAnimation, ScrollView, StyleSheet, View} from 'react-native'
-import {useSafeAreaInsets} from 'react-native-safe-area-context'
+import {
+  useSafeAreaFrame,
+  useSafeAreaInsets,
+} from 'react-native-safe-area-context'
 import {BlurView} from 'expo-blur'
 import {useLingui} from '@lingui/react/macro'
 
@@ -17,10 +20,16 @@ export function Footer({altText, isAltExpanded, onToggleAltExpanded}: Props) {
   const {t: l} = useLingui()
   const t = useTheme()
   const insets = useSafeAreaInsets()
+  const {height: screenHeight} = useSafeAreaFrame()
   const isMomentumScrolling = useRef(false)
 
   if (!altText) return null
 
+  // Cap the overlay height so long alt text - or text enlarged by the OS via
+  // Dynamic Type / font scaling - scrolls within the overlay instead of growing
+  // past the top of the screen. Leaves the upper half clear for the header.
+  const maxHeight = screenHeight / 2
+
   return (
     <View
       style={[
@@ -46,6 +55,7 @@ export function Footer({altText, isAltExpanded, onToggleAltExpanded}: Props) {
             }),
           ]}>
           <ScrollView
+            style={{maxHeight}}
             scrollEnabled={isAltExpanded}
             onMomentumScrollBegin={() => {
               isMomentumScrolling.current = true