diff --git a/widget/cocoa/nsChildView.h b/widget/cocoa/nsChildView.h
index 07be602f7b33..55b41d73209a 100644
--- a/widget/cocoa/nsChildView.h
+++ b/widget/cocoa/nsChildView.h
@@ -156,7 +156,9 @@ class WidgetRenderingContext;
   BOOL mExpectingWheelStop;
 
   // Set to YES when our GL surface has been updated and we need to call
-  // updateGLContext before we composite.
+  // updateGLContext on the compositor thread before we composite.
+  // Accesses from different threads are synchronized via mGLContext's
+  // CGLContextObj lock.
   BOOL mNeedsGLUpdate;
 
   // Holds our drag service across multiple drag calls. The reference to the
@@ -166,6 +168,7 @@ class WidgetRenderingContext;
   // when handling |draggingUpdated:| messages.
   nsIDragService* mDragService;
 
+  // The NSOpenGLContext that is attached to our mPixelHostingView.
   NSOpenGLContext* mGLContext;
 
   // Gestures support
diff --git a/widget/cocoa/nsChildView.mm b/widget/cocoa/nsChildView.mm
index 249ea2da93f7..74da6e2f5727 100644
--- a/widget/cocoa/nsChildView.mm
+++ b/widget/cocoa/nsChildView.mm
@@ -2802,7 +2802,6 @@ class WidgetsReleaserRunnable final : public mozilla::Runnable {
 }
 
 @end
-;
 
 @implementation ChildView
 
@@ -2994,14 +2993,14 @@ NSEvent* gLastDragMouseDownEvent = nil;
     return false;
   }
 
+  CGLLockContext((CGLContextObj)[aGLContext CGLContextObj]);
+
   if (!mGLContext) {
     mGLContext = aGLContext;
     [mGLContext retain];
-    mNeedsGLUpdate = true;
+    mNeedsGLUpdate = YES;
   }
 
-  CGLLockContext((CGLContextObj)[aGLContext CGLContextObj]);
-
   if (mNeedsGLUpdate) {
     [self updateGLContext];
     mNeedsGLUpdate = NO;