Bug 1674317 - Load about:home scripts via the subscript loader when hitting the about:home startup cache. r=k88hudson

Using the subscript loader is currently our best way of getting the loaded scripts recorded for entry
in the ScriptPreloader which is used to cache startup-related JavaScript within the startup window.

Originally, we were using <script> tags to load those scripts, but unfortunately, that loading mechanism
does not qualify for being noted by the ScriptPreloader.

This is a workaround until the Stencil project by the SpiderMonkey team Phase 2 is complete (see bug 1663956),
which should allow us to more easily note scripts to cache at runtime.

Differential Revision: https://phabricator.services.mozilla.com/D95407

browser/actors/AboutNewTabChild.jsm

@@ -48,17 +48,6 @@ class AboutNewTabChild extends JSWindowActorChild {
         return;
       }
 
-      // In the event that the document that was loaded here was the cached
-      // about:home document, then there's nothing further to do - the page
-      // will load its scripts itself.
-      //
-      // Note that it's okay to waive the xray wrappers here since this actor
-      // is registered to only run in the privileged about content process from
-      // about:home, about:newtab and about:welcome.
-      if (ChromeUtils.waiveXrays(this.contentWindow).__FROM_STARTUP_CACHE__) {
-        return;
-      }
-
       const debug = !AppConstants.RELEASE_OR_BETA && ACTIVITY_STREAM_DEBUG;
       const debugString = debug ? "-dev" : "";
 

browser/components/newtab/data/content/abouthomecache/page.html.template

@@ -38,16 +38,6 @@
       {{ MARKUP }}
     </div>
     <div id="footer-asrouter-container" role="presentation"></div>
-    <script src="chrome://browser/content/contentSearchUI.js"></script>
-    <script src="chrome://browser/content/contentSearchHandoffUI.js"></script>
-    <script src="chrome://browser/content/contentTheme.js"></script>
-    <script src="resource://activity-stream/vendor/react.js"></script>
-    <script src="resource://activity-stream/vendor/react-dom.js"></script>
-    <script src="resource://activity-stream/vendor/prop-types.js"></script>
-    <script src="resource://activity-stream/vendor/redux.js"></script>
-    <script src="resource://activity-stream/vendor/react-redux.js"></script>
-    <script src="resource://activity-stream/vendor/react-transition-group.js"></script>
-    <script src="resource://activity-stream/data/content/activity-stream.bundle.js"></script>
     <script src="about:home?jscache"></script>
   </body>
 </html>

browser/components/newtab/data/content/abouthomecache/script.js.template

@@ -15,4 +15,5 @@
 #
 #endif
 window.__FROM_STARTUP_CACHE__ = true;
-window.NewtabRenderUtils.renderCache({{ STATE }});
+window.__STARTUP_STATE__ = {{ STATE }};
+

browser/components/newtab/data/content/newtab-render.js

@@ -4,4 +4,8 @@
 "use strict";
 
 // exported by activity-stream.bundle.js
-window.NewtabRenderUtils.renderWithoutState();
+if (window.__FROM_STARTUP_CACHE__) {
+  window.NewtabRenderUtils.renderCache(window.__STARTUP_STATE__);
+} else {
+  window.NewtabRenderUtils.renderWithoutState();
+}

browser/components/newtab/test/xpcshell/test_AboutHomeStartupCacheWorker.js

@@ -40,6 +40,8 @@ ChromeUtils.defineModuleGetter(
 XPCOMUtils.defineLazyGlobalGetters(this, ["DOMParser"]);
 
 const CACHE_WORKER_URL = "resource://activity-stream/lib/cache-worker.js";
+const NEWTAB_RENDER_URL =
+  "resource://activity-stream/data/content/newtab-render.js";
 
 /**
  * In order to make this test less brittle, much of Activity Stream is
@@ -175,6 +177,11 @@ add_task(async function test_cache_worker() {
     },
   };
   Cu.evalInSandbox(script, sandbox);
+
+  // The NEWTAB_RENDER_URL script is what ultimately causes the state
+  // to be passed into the renderCache function.
+  Services.scriptloader.loadSubScript(NEWTAB_RENDER_URL, sandbox);
+
   equal(
     sandbox.window.__FROM_STARTUP_CACHE__,
     true,