Bug 1940688 - Add a pref to make top level pages always active for the purpose of refresh driver throttling and such. r=layout-reviewers,tnikkel

Differential Revision: https://phabricator.services.mozilla.com/D233937

dom/base/Document.cpp

@@ -7364,7 +7364,7 @@ bool Document::ShouldThrottleFrameRequests() const {
     return false;
   }
 
-  if (Hidden()) {
+  if (Hidden() && !StaticPrefs::layout_testing_top_level_always_active()) {
     // We're not visible (probably in a background tab or the bf cache).
     return true;
   }

layout/base/PresShell.cpp

@@ -11246,6 +11246,12 @@ bool PresShell::ComputeActiveness() const {
   MOZ_LOG(gLog, LogLevel::Debug,
           (" > BrowsingContext %p  active: %d", bc, inActiveTab));
 
+  if (StaticPrefs::layout_testing_top_level_always_active() && bc &&
+      bc->IsTop()) {
+    MOZ_LOG(gLog, LogLevel::Debug, (" > Activeness overridden by pref"));
+    return true;
+  }
+
   Document* root = nsContentUtils::GetInProcessSubtreeRootDocument(doc);
   if (auto* browserChild = BrowserChild::GetFrom(root->GetDocShell())) {
     // We might want to activate a tab even though the browsing-context is not

modules/libpref/init/StaticPrefList.yaml

@@ -10040,6 +10040,15 @@
   value: true
   mirror: always
 
+# Treat top level pages as always-active for the purpose of refresh driver
+# throttling and such.
+#
+# Intended for testing automation and so forth.
+- name: layout.testing.top-level-always-active
+  type: bool
+  value: false
+  mirror: always
+
 # Pref to stop overlay scrollbars from fading out, for testing purposes.
 - name: layout.testing.overlay-scrollbars.always-visible
   type: bool