Bug 1491442 - Fix up and document mNeedsGLUpdate locking semantics, and remove a stray semicolon. r=mattwoodrow

Differential Revision: https://phabricator.services.mozilla.com/D38748

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

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

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;