Android lightbox minor perf improvement (#9274)

* minor perf improvement on android for lightbox

* Use correct `cachePolicy` when prefetching lightbox images (#9275)

* use `memory` cachePolicy when prefetching

* use `memory` cache policy on iOS lightbox images

src/components/Post/Embed/ImageEmbed.tsx

@@ -64,7 +64,10 @@ export function ImageEmbed({
     }
     const onPressIn = (_: number) => {
       InteractionManager.runAfterInteractions(() => {
-        Image.prefetch(items.map(i => i.uri))
+        Image.prefetch(
+          items.map(i => i.uri),
+          'memory',
+        )
       })
     }
 

src/view/com/lightbox/ImageViewing/components/ImageItem/ImageItem.ios.tsx

@@ -252,6 +252,7 @@ const ImageItem = ({
                       onLoad({width: e.source.width, height: e.source.height})
                     }
               }
+              cachePolicy="memory"
             />
           </Animated.View>
         </Animated.View>

src/view/com/util/List.tsx

@@ -39,7 +39,7 @@ export type ListProps<ItemT = any> = Omit<
   sideBorders?: boolean
   progressViewOffset?: number
 }
-export type ListRef = React.MutableRefObject<FlatList_INTERNAL | null>
+export type ListRef = React.RefObject<FlatList_INTERNAL | null>
 
 const SCROLLED_DOWN_LIMIT = 200
 
@@ -61,7 +61,7 @@ let List = React.forwardRef<ListMethods, ListProps>(
     const isScrolledDown = useSharedValue(false)
     const t = useTheme()
     const dedupe = useDedupe(400)
-    const {activeLightbox} = useLightbox()
+    const scrollsToTop = useAllowScrollToTop()
 
     function handleScrolledDownChange(didScrollDown: boolean) {
       onScrolledDownChange?.(didScrollDown)
@@ -168,7 +168,7 @@ let List = React.forwardRef<ListMethods, ListProps>(
         contentOffset={contentOffset}
         refreshControl={refreshControl}
         onScroll={scrollHandler}
-        scrollsToTop={!activeLightbox}
+        scrollsToTop={scrollsToTop}
         scrollEventThrottle={1}
         style={style}
         // @ts-expect-error FlatList_INTERNAL ref type is wrong -sfn
@@ -181,3 +181,11 @@ List.displayName = 'List'
 
 List = memo(List)
 export {List}
+
+// We only want to use this context value on iOS because the `scrollsToTop` prop is iOS-only
+// removing it saves us a re-render on Android
+const useAllowScrollToTop = isIOS ? useAllowScrollToTopIOS : () => undefined
+function useAllowScrollToTopIOS() {
+  const {activeLightbox} = useLightbox()
+  return !activeLightbox
+}