Bug 1342927 - Add a hidden pref to disable force painting of tabs so that browser_tabSpinnerProbe.js can pass. r=billm

MozReview-Commit-ID: 4q2dSBmvVV9

--HG--
extra : rebase_source : 70fcc277918498fa71046b9d57419781be55bcf8

browser/base/content/test/tabs/browser_tabSpinnerProbe.js

@@ -85,7 +85,15 @@ function* testProbe(aProbe) {
 
 add_task(function* setup() {
   yield SpecialPowers.pushPrefEnv({
-    set: [["dom.ipc.processCount", 1]]
+    set: [
+      ["dom.ipc.processCount", 1],
+      // We can interrupt JS to paint now, which is great for
+      // users, but bad for testing spinners. We temporarily
+      // disable that feature for this test so that we can
+      // easily get ourselves into a predictable tab spinner
+      // state.
+      ["browser.tabs.remote.force-paint", false],
+    ]
   });
 });
 

dom/ipc/ProcessHangMonitor.cpp

@@ -275,8 +275,12 @@ private:
   nsDataHashtable<nsUint32HashKey, nsString> mBrowserCrashDumpIds;
   Mutex mBrowserCrashDumpHashLock;
   mozilla::ipc::TaskFactory<HangMonitorParent> mMainThreadTaskFactory;
+
+  static bool sShouldForcePaint;
 };
 
+bool HangMonitorParent::sShouldForcePaint = true;
+
 } // namespace
 
 /* HangMonitorChild implementation */
@@ -575,6 +579,13 @@ HangMonitorParent::HangMonitorParent(ProcessHangMonitor* aMonitor)
 {
   MOZ_RELEASE_ASSERT(NS_IsMainThread());
   mReportHangs = mozilla::Preferences::GetBool("dom.ipc.reportProcessHangs", false);
+
+  static bool sInited = false;
+  if (!sInited) {
+    sInited = true;
+    Preferences::AddBoolVarCache(&sShouldForcePaint,
+                                 "browser.tabs.remote.force-paint", true);
+  }
 }
 
 HangMonitorParent::~HangMonitorParent()
@@ -632,9 +643,11 @@ void
 HangMonitorParent::ForcePaint(dom::TabParent* aTab, uint64_t aLayerObserverEpoch)
 {
   MOZ_RELEASE_ASSERT(NS_IsMainThread());
-  TabId id = aTab->GetTabId();
-  MonitorLoop()->PostTask(NewNonOwningRunnableMethod<TabId, uint64_t>(
-                            this, &HangMonitorParent::ForcePaintOnThread, id, aLayerObserverEpoch));
+  if (sShouldForcePaint) {
+    TabId id = aTab->GetTabId();
+    MonitorLoop()->PostTask(NewNonOwningRunnableMethod<TabId, uint64_t>(
+                              this, &HangMonitorParent::ForcePaintOnThread, id, aLayerObserverEpoch));
+  }
 }
 
 void