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/state/shell/minimal-mode.tsx

@@ -6,32 +6,55 @@ import {
   withSpring,
 } from 'react-native-reanimated'
 
-type StateContext = SharedValue<number>
+type StateContext = {
+  headerMode: SharedValue<number>
+  footerMode: SharedValue<number>
+}
 type SetContext = (v: boolean) => void
 
 const stateContext = React.createContext<StateContext>({
-  value: 0,
-  addListener() {},
-  removeListener() {},
-  modify() {},
+  headerMode: {
+    value: 0,
+    addListener() {},
+    removeListener() {},
+    modify() {},
+  },
+  footerMode: {
+    value: 0,
+    addListener() {},
+    removeListener() {},
+    modify() {},
+  },
 })
 const setContext = React.createContext<SetContext>((_: boolean) => {})
 
 export function Provider({children}: React.PropsWithChildren<{}>) {
-  const mode = useSharedValue(0)
+  const headerMode = useSharedValue(0)
+  const footerMode = useSharedValue(0)
   const setMode = React.useCallback(
     (v: boolean) => {
       'worklet'
       // Cancel any existing animation
-      cancelAnimation(mode)
-      mode.value = withSpring(v ? 1 : 0, {
+      cancelAnimation(headerMode)
+      headerMode.value = withSpring(v ? 1 : 0, {
+        overshootClamping: true,
+      })
+      cancelAnimation(footerMode)
+      footerMode.value = withSpring(v ? 1 : 0, {
         overshootClamping: true,
       })
     },
-    [mode],
+    [headerMode, footerMode],
+  )
+  const value = React.useMemo(
+    () => ({
+      headerMode,
+      footerMode,
+    }),
+    [headerMode, footerMode],
   )
   return (
-    <stateContext.Provider value={mode}>
+    <stateContext.Provider value={value}>
       <setContext.Provider value={setMode}>{children}</setContext.Provider>
     </stateContext.Provider>
   )