Adjust FlatList performance in main feeds (#3134)

* adjust flatlist perf settings

* calculate initial num to render based on screen height

* adjust window size

* don't react to screen height changes

src/lib/hooks/useInitialNumToRender.ts

···
+import React from 'react'
+import {Dimensions} from 'react-native'
+
+const MIN_POST_HEIGHT = 100
+
+export function useInitialNumToRender(minItemHeight: number = MIN_POST_HEIGHT) {
+  return React.useMemo(() => {
+    const screenHeight = Dimensions.get('window').height
+    return Math.ceil(screenHeight / minItemHeight) + 1
+  }, [minItemHeight])
+}

src/screens/Hashtag.tsx

···
 import {shareUrl} from 'lib/sharing'
 import {HITSLOP_10} from 'lib/constants'
 import {isNative} from 'platform/detection'
+import {useInitialNumToRender} from 'lib/hooks/useInitialNumToRender'
 
 const renderItem = ({item}: ListRenderItemInfo<PostView>) => {
   return <Post post={item} />
···
   const {tag, author} = route.params
   const setMinimalShellMode = useSetMinimalShellMode()
   const {_} = useLingui()
+  const initialNumToRender = useInitialNumToRender()
   const [isPTR, setIsPTR] = React.useState(false)
 
   const fullTag = React.useMemo(() => {
···
               onRetry={fetchNextPage}
             />
           }
+          initialNumToRender={initialNumToRender}
+          windowSize={11}
         />
       )}
     </>

src/view/com/posts/Feed.tsx

···
 import {useLingui} from '@lingui/react'
 import {DiscoverFallbackHeader} from './DiscoverFallbackHeader'
 import {FALLBACK_MARKER_POST} from '#/lib/api/feed/home'
+import {useInitialNumToRender} from 'lib/hooks/useInitialNumToRender'
 
 const LOADING_ITEM = {_reactKey: '__loading__'}
 const EMPTY_FEED_ITEM = {_reactKey: '__empty__'}
···
   const {_} = useLingui()
   const queryClient = useQueryClient()
   const {currentAccount} = useSession()
+  const initialNumToRender = useInitialNumToRender()
   const [isPTRing, setIsPTRing] = React.useState(false)
   const checkForNewRef = React.useRef<(() => void) | null>(null)
   const lastFetchRef = React.useRef<number>(Date.now())
···
         desktopFixedHeight={
           desktopFixedHeightOffset ? desktopFixedHeightOffset : true
         }
+        initialNumToRender={initialNumToRender}
+        windowSize={11}
       />
     </View>
   )