Bug 1718755 - Use an early refresh driver runner rather than Dispatch() to coalesce theme changes. r=stransky

That way we guarantee they are processed before display.

Differential Revision: https://phabricator.services.mozilla.com/D119023

layout/base/nsPresContext.cpp

@@ -586,18 +586,34 @@ void nsPresContext::PreferenceChanged(const char* aPrefName) {
   }
 }
 
+struct WeakRunnableMethod : Runnable {
+  using Method = void (nsPresContext::*)();
+
+  WeakRunnableMethod(const char* aName, nsPresContext* aPc, Method aMethod)
+      : Runnable(aName), mPresContext(aPc), mMethod(aMethod) {}
+
+  NS_IMETHOD Run() override {
+    if (nsPresContext* pc = mPresContext.get()) {
+      (pc->*mMethod)();
+    }
+    return NS_OK;
+  }
+
+ private:
+  WeakPtr<nsPresContext> mPresContext;
+  Method mMethod;
+};
+
 void nsPresContext::DispatchPrefChangedRunnableIfNeeded() {
   if (mPostedPrefChangedRunnable) {
     return;
   }
 
-  nsCOMPtr<nsIRunnable> runnable =
-      NewRunnableMethod("nsPresContext::UpdateAfterPreferencesChanged", this,
+  nsCOMPtr<nsIRunnable> runnable = new WeakRunnableMethod(
+      "nsPresContext::UpdateAfterPreferencesChanged", this,
       &nsPresContext::UpdateAfterPreferencesChanged);
-  nsresult rv = Document()->Dispatch(TaskCategory::Other, runnable.forget());
-  if (NS_SUCCEEDED(rv)) {
+  RefreshDriver()->AddEarlyRunner(runnable);
   mPostedPrefChangedRunnable = true;
-  }
 }
 
 void nsPresContext::UpdateAfterPreferencesChanged() {
@@ -1361,13 +1377,11 @@ void nsPresContext::ThemeChanged(widget::ThemeChangeKind aKind) {
     sThemeChanged = true;
 
     nsCOMPtr<nsIRunnable> ev =
-        NewRunnableMethod("nsPresContext::ThemeChangedInternal", this,
+        new WeakRunnableMethod("nsPresContext::ThemeChangedInternal", this,
                                &nsPresContext::ThemeChangedInternal);
-    nsresult rv = Document()->Dispatch(TaskCategory::Other, ev.forget());
-    if (NS_SUCCEEDED(rv)) {
+    RefreshDriver()->AddEarlyRunner(ev);
     mPendingThemeChanged = true;
   }
-  }
 }
 
 void nsPresContext::ThemeChangedInternal() {

layout/base/nsRefreshDriver.cpp

@@ -2885,6 +2885,8 @@ void nsRefreshDriver::Disconnect() {
 
   StopTimer();
 
+  mEarlyRunners.Clear();
+
   if (mPresContext) {
     mPresContext = nullptr;
     if (--sRefreshDriverCount == 0) {