Update theme on pageshow event

When a user goes forward or back, the page may be rendered from the
back/forward cache (https://web.dev/bfcache/) rather than from scratch. If
they have changed theme in the meantime, that means seeing an incorrect
theme on the page they went forward or back to. The `pageshow` event
fires on such navigations, so we can update the theme based on that event.

Author: jsha

src/librustdoc/html/static/js/storage.js

@@ -216,6 +216,15 @@ var updateSystemTheme = (function() {
     };
 })();
 
+function switchToSavedTheme() {
+    switchTheme(
+        window.currentTheme,
+        window.mainTheme,
+        getSettingValue("theme") || "light",
+        false
+    );
+}
+
 if (getSettingValue("use-system-theme") !== "false" && window.matchMedia) {
     // update the preferred dark theme if the user is already using a dark theme
     // See https://github.com/rust-lang/rust/pull/77809#issuecomment-707875732
@@ -228,10 +237,20 @@ if (getSettingValue("use-system-theme") !== "false" && window.matchMedia) {
     // call the function to initialize the theme at least once!
     updateSystemTheme();
 } else {
-    switchTheme(
-        window.currentTheme,
-        window.mainTheme,
-        getSettingValue("theme") || "light",
-        false
-    );
+    switchToSavedTheme();
 }
+
+// If we navigate away (for example to a settings page), and then use the back or
+// forward button to get back to a page, the theme may have changed in the meantime.
+// But scripts may not be re-loaded in such a case due to the bfcache
+// (https://web.dev/bfcache/). The "pageshow" event triggers on such navigations.
+// Use that opportunity to update the theme.
+// We use a setTimeout with a 0 timeout here to put the change on the event queue.
+// For some reason, if we try to change the theme while the `pageshow` event is
+// running, it sometimes fails to take effect. The problem manifests on Chrome,
+// specifically when talking to a remote website with no caching.
+window.addEventListener("pageshow", function(ev) {
+    if (ev.persisted) {
+        setTimeout(switchToSavedTheme, 0);
+    }
+});