Keep pager feeds in sync with the right pane (#2775)

* Hoist selected feed state

* Seed state from params

* Refine and fix logic

* Fix scroll restoration

* Soft reset on second click

src/state/shell/selected-feed.tsx

@@ -0,0 +1,61 @@
+import React from 'react'
+import * as persisted from '#/state/persisted'
+import {isWeb} from '#/platform/detection'
+
+type StateContext = string
+type SetContext = (v: string) => void
+
+const stateContext = React.createContext<StateContext>('home')
+const setContext = React.createContext<SetContext>((_: string) => {})
+
+function getInitialFeed() {
+  if (isWeb) {
+    if (window.location.pathname === '/') {
+      const params = new URLSearchParams(window.location.search)
+      const feedFromUrl = params.get('feed')
+      if (feedFromUrl) {
+        // If explicitly booted from a link like /?feed=..., prefer that.
+        return feedFromUrl
+      }
+    }
+    const feedFromSession = sessionStorage.getItem('lastSelectedHomeFeed')
+    if (feedFromSession) {
+      // Fall back to a previously chosen feed for this browser tab.
+      return feedFromSession
+    }
+  }
+  const feedFromPersisted = persisted.get('lastSelectedHomeFeed')
+  if (feedFromPersisted) {
+    // Fall back to the last chosen one across all tabs.
+    return feedFromPersisted
+  }
+  return 'home'
+}
+
+export function Provider({children}: React.PropsWithChildren<{}>) {
+  const [state, setState] = React.useState(getInitialFeed)
+
+  const saveState = React.useCallback((feed: string) => {
+    setState(feed)
+    if (isWeb) {
+      try {
+        sessionStorage.setItem('lastSelectedHomeFeed', feed)
+      } catch {}
+    }
+    persisted.write('lastSelectedHomeFeed', feed)
+  }, [])
+
+  return (
+    <stateContext.Provider value={state}>
+      <setContext.Provider value={saveState}>{children}</setContext.Provider>
+    </stateContext.Provider>
+  )
+}
+
+export function useSelectedFeed() {
+  return React.useContext(stateContext)
+}
+
+export function useSetSelectedFeed() {
+  return React.useContext(setContext)
+}