Fix fixed footer experiment (#4969)

* Split minimal shell mode into headerMode and footerMode

For now, we'll always write them in sync. When we read them, we'll use headerMode as source of truth. This will let us keep footerMode independent in a future commit.

* Remove fixed_bottom_bar special cases during calculation

This isn't the right time to determine special behavior. Instead we'll adjust footerMode itself conditionally on the gate.

* Copy-paste setMode into MainScrollProvider

This lets us fork the implementation later just for this case.

* Gate footer adjustment in MainScrollProvider

This is the final piece. Normal calls to setMode() keep setting both header and footer, but MainScrollProvider adjusts the footer conditionally.

src/view/com/util/MainScrollProvider.tsx

@@ -4,11 +4,13 @@ import {
   cancelAnimation,
   interpolate,
   useSharedValue,
+  withSpring,
 } from 'react-native-reanimated'
 import EventEmitter from 'eventemitter3'
 
 import {ScrollProvider} from '#/lib/ScrollContext'
-import {useMinimalShellMode, useSetMinimalShellMode} from '#/state/shell'
+import {useGate} from '#/lib/statsig/statsig'
+import {useMinimalShellMode} from '#/state/shell'
 import {useShellLayout} from '#/state/shell/shell-layout'
 import {isNative, isWeb} from 'platform/detection'
 
@@ -21,11 +23,29 @@ function clamp(num: number, min: number, max: number) {
 
 export function MainScrollProvider({children}: {children: React.ReactNode}) {
   const {headerHeight} = useShellLayout()
-  const mode = useMinimalShellMode()
-  const setMode = useSetMinimalShellMode()
+  const {headerMode, footerMode} = useMinimalShellMode()
   const startDragOffset = useSharedValue<number | null>(null)
   const startMode = useSharedValue<number | null>(null)
   const didJustRestoreScroll = useSharedValue<boolean>(false)
+  const gate = useGate()
+  const isFixedBottomBar = gate('fixed_bottom_bar')
+
+  const setMode = React.useCallback(
+    (v: boolean) => {
+      'worklet'
+      cancelAnimation(headerMode)
+      headerMode.value = withSpring(v ? 1 : 0, {
+        overshootClamping: true,
+      })
+      if (!isFixedBottomBar) {
+        cancelAnimation(footerMode)
+        footerMode.value = withSpring(v ? 1 : 0, {
+          overshootClamping: true,
+        })
+      }
+    },
+    [headerMode, footerMode, isFixedBottomBar],
+  )
 
   useEffect(() => {
     if (isWeb) {
@@ -55,11 +75,11 @@ export function MainScrollProvider({children}: {children: React.ReactNode}) {
           setMode(true)
         } else {
           // Snap to whichever state is the closest.
-          setMode(Math.round(mode.value) === 1)
+          setMode(Math.round(headerMode.value) === 1)
         }
       }
     },
-    [startDragOffset, startMode, setMode, mode, headerHeight],
+    [startDragOffset, startMode, setMode, headerMode, headerHeight],
   )
 
   const onBeginDrag = useCallback(
@@ -67,10 +87,10 @@ export function MainScrollProvider({children}: {children: React.ReactNode}) {
       'worklet'
       if (isNative) {
         startDragOffset.value = e.contentOffset.y
-        startMode.value = mode.value
+        startMode.value = headerMode.value
       }
     },
-    [mode, startDragOffset, startMode],
+    [headerMode, startDragOffset, startMode],
   )
 
   const onEndDrag = useCallback(
@@ -102,7 +122,10 @@ export function MainScrollProvider({children}: {children: React.ReactNode}) {
       'worklet'
       if (isNative) {
         if (startDragOffset.value === null || startMode.value === null) {
-          if (mode.value !== 0 && e.contentOffset.y < headerHeight.value) {
+          if (
+            headerMode.value !== 0 &&
+            e.contentOffset.y < headerHeight.value
+          ) {
             // If we're close enough to the top, always show the shell.
             // Even if we're not dragging.
             setMode(false)
@@ -119,11 +142,15 @@ export function MainScrollProvider({children}: {children: React.ReactNode}) {
           [-1, 1],
         )
         const newValue = clamp(startMode.value + dProgress, 0, 1)
-        if (newValue !== mode.value) {
+        if (newValue !== headerMode.value) {
           // Manually adjust the value. This won't be (and shouldn't be) animated.
           // Cancel any any existing animation
-          cancelAnimation(mode)
-          mode.value = newValue
+          cancelAnimation(headerMode)
+          headerMode.value = newValue
+          if (!isFixedBottomBar) {
+            cancelAnimation(footerMode)
+            footerMode.value = newValue
+          }
         }
       } else {
         if (didJustRestoreScroll.value) {
@@ -145,11 +172,13 @@ export function MainScrollProvider({children}: {children: React.ReactNode}) {
     },
     [
       headerHeight,
-      mode,
+      headerMode,
+      footerMode,
       setMode,
       startDragOffset,
       startMode,
       didJustRestoreScroll,
+      isFixedBottomBar,
     ],
   )
 

src/view/screens/Home.tsx

@@ -167,7 +167,7 @@ function HomeScreenReady({
     }),
   )
 
-  const mode = useMinimalShellMode()
+  const {footerMode} = useMinimalShellMode()
   const {isMobile} = useWebMediaQueries()
   useFocusEffect(
     React.useCallback(() => {
@@ -177,7 +177,7 @@ function HomeScreenReady({
       }
       const listener = AppState.addEventListener('change', nextAppState => {
         if (nextAppState === 'active') {
-          if (isMobile && mode.value === 1) {
+          if (isMobile && footerMode.value === 1) {
             // Reveal the bottom bar so you don't miss notifications or messages.
             // TODO: Experiment with only doing it when unread > 0.
             setMinimalShellMode(false)
@@ -187,7 +187,7 @@ function HomeScreenReady({
       return () => {
         listener.remove()
       }
-    }, [setMinimalShellMode, mode, isMobile, gate]),
+    }, [setMinimalShellMode, footerMode, isMobile, gate]),
   )
 
   const onPageSelected = React.useCallback(