Bug 1570797, Add a pref to control the time when certain tasks are deprioritized during page load, r=farre

Differential Revision: https://phabricator.services.mozilla.com/D40330

--HG--
extra : moz-landing-system : lando

dom/base/Document.cpp

@@ -34,6 +34,7 @@
 #include "mozilla/StaticPrefs_dom.h"
 #include "mozilla/StaticPrefs_full_screen_api.h"
 #include "mozilla/StaticPrefs_layout.h"
+#include "mozilla/StaticPrefs_page_load.h"
 #include "mozilla/StaticPrefs_plugins.h"
 #include "mozilla/StaticPrefs_privacy.h"
 #include "mozilla/StaticPrefs_security.h"
@@ -15851,7 +15852,8 @@ bool Document::HasRecentlyStartedForegroundLoads() {
       nsPIDOMWindowInner* win = doc->GetInnerWindow();
       if (win) {
         Performance* perf = win->GetPerformance();
-        if (perf && perf->Now() < 5000) {
+        if (perf &&
+            perf->Now() < StaticPrefs::page_load_deprioritization_period()) {
           return true;
         }
       }

dom/base/nsGlobalWindowInner.cpp

@@ -110,6 +110,7 @@
 #include "mozilla/dom/DocGroup.h"
 #include "mozilla/dom/TabGroup.h"
 #include "mozilla/StaticPrefs_dom.h"
+#include "mozilla/StaticPrefs_page_load.h"
 #include "PaintWorkletImpl.h"
 
 // Interfaces Needed
@@ -2575,8 +2576,9 @@ void nsGlobalWindowInner::AddDeprioritizedLoadRunner(nsIRunnable* aRunner) {
   MOZ_ASSERT(GetWindowForDeprioritizedLoadRunner() == this);
   RefPtr<DeprioritizedLoadRunner> runner = new DeprioritizedLoadRunner(aRunner);
   mDeprioritizedLoadRunner.insertBack(runner);
-  NS_DispatchToCurrentThreadQueue(runner.forget(), 5000,
-                                  EventQueuePriority::Idle);
+  NS_DispatchToCurrentThreadQueue(
+      runner.forget(), StaticPrefs::page_load_deprioritization_period(),
+      EventQueuePriority::Idle);
 }
 
 // nsISpeechSynthesisGetter

layout/base/nsRefreshDriver.cpp

@@ -51,6 +51,7 @@
 #include "mozilla/StaticPrefs_apz.h"
 #include "mozilla/StaticPrefs_gfx.h"
 #include "mozilla/StaticPrefs_layout.h"
+#include "mozilla/StaticPrefs_page_load.h"
 #include "nsViewManager.h"
 #include "GeckoProfiler.h"
 #include "nsNPAPIPluginInstance.h"
@@ -601,7 +602,9 @@ class VsyncRefreshDriverTimer : public RefreshDriverTimer {
               dom::Performance* perf = win->GetPerformance();
               // Limit slower refresh rate to 5 seconds between the
               // first contentful paint and page load.
-              if (perf && perf->Now() < 5000) {
+              if (perf &&
+                  perf->Now() <
+                      StaticPrefs::page_load_deprioritization_period()) {
                 if (mProcessedVsync) {
                   mProcessedVsync = false;
                   // Handle this case similarly to the code above, but just

modules/libpref/init/StaticPrefList.yaml

@@ -5526,6 +5526,18 @@
   value: false
   mirror: always
 
+
+#---------------------------------------------------------------------------
+# Prefs starting with "page_load."
+#---------------------------------------------------------------------------
+
+# Time in milliseconds during which certain tasks are deprioritized during
+# page load.
+- name: page_load.deprioritization_period
+  type: uint32_t
+  value: 5000
+  mirror: always
+
 #---------------------------------------------------------------------------
 # Prefs starting with "plain_text."
 #---------------------------------------------------------------------------

modules/libpref/moz.build

@@ -56,6 +56,7 @@ pref_groups = [
     'mousewheel',
     'network',
     'nglayout',
+    'page_load',
     'plain_text',
     'plugins',
     'preferences',