fix: an open popup follows a network switch the background saves (closes #650)

When the background saves a network switch after the user approves a
site's request, or another popup page saves one, an open popup now
notices the stored record change and takes the new network from it,
holding the lock, inside a save. It then leaves the page as a switch in
Settings does: the stack is cut, every screen counts as not yet
rendered, and the screen on top is drawn again. The cut and the redraw
are now two functions in the Settings view that both paths call. A
record already on the popup's network, such as its own Settings switch,
changes nothing. The popup never switches the network itself.

Model: opus-5-5

src/shared/browserApi.js

@@ -224,6 +224,19 @@ function storageRemove(keys) {
     return Promise.resolve(storage.remove(keys));
 }
 
+/**
+ * Call `listener` after every write to `storage.local`, whether this page,
+ * another page or the background made it, with what the write changed, as the
+ * browser reports it: `{ [key]: { oldValue, newValue } }`.
+ *
+ * @param {function(Object): void} listener
+ */
+function onStorageChanged(listener) {
+    namespaceMember("storage").onChanged.addListener((changes, areaName) => {
+        if (areaName === "local") listener(changes);
+    });
+}
+
 /**
  * Run `fn` while holding the browser's lock called `name`
  * (`navigator.locks`), and settle as `fn` settles. Every page of the extension
@@ -385,6 +398,7 @@ module.exports = {
     getPlatformInfo,
     hasBrowserNamespace,
     notify,
+    onStorageChanged,
     runtimeApi,
     sendMessage,
     sessionStorageGet,