Bug 1772463 - Move mVsyncSkipCounter into mState. r=bas

VsyncDispatcher::NotifyVsync can be called from two different threads
at the same time, if it just swapped out its vsync source and the old
vsync source is still notifying it. So we need to protect mVsyncSkipCounter
behind a lock.

Differential Revision: https://phabricator.services.mozilla.com/D148958

widget/VsyncDispatcher.cpp

@@ -126,17 +126,20 @@ TimeDuration VsyncDispatcher::GetVsyncRate() {
 }
 
 void VsyncDispatcher::NotifyVsync(const VsyncEvent& aVsync) {
-  if (++mVsyncSkipCounter < StaticPrefs::gfx_display_frame_rate_divisor()) {
-    return;
-  }
-  mVsyncSkipCounter = 0;
-
   nsTArray<RefPtr<VsyncObserver>> observers;
   bool shouldDispatchToMainThread = false;
   {
+    auto state = mState.Lock();
+
+    // Respect the pref gfx.display.frame-rate-divisor.
+    if (++state->mVsyncSkipCounter <
+        StaticPrefs::gfx_display_frame_rate_divisor()) {
+      return;
+    }
+    state->mVsyncSkipCounter = 0;
+
     // Copy out the observers so that we don't keep the mutex
     // locked while notifying vsync.
-    auto state = mState.Lock();
     observers = state->mObservers.Clone();
     shouldDispatchToMainThread = !state->mMainThreadObservers.IsEmpty() &&
                                  (state->mLastVsyncIdSentToMainThread ==

widget/VsyncDispatcher.h

@@ -141,11 +141,15 @@ class VsyncDispatcher final {
     // Always non-null.
     RefPtr<gfx::VsyncSource> mCurrentVsyncSource;
 
+    // The number of skipped vsyncs since the last non-skipped vsync.
+    // Reset to zero every n vsyncs, where n is given by the pref
+    // gfx.display.frame-rate-divisor.
+    int32_t mVsyncSkipCounter = 0;
+
     bool mIsObservingVsync = false;
   };
 
   DataMutex<State> mState;
-  int32_t mVsyncSkipCounter = 0;
 };
 
 }  // namespace mozilla