--- a/widget/cocoa/nsCocoaWindow.mm
+++ b/widget/cocoa/nsCocoaWindow.mm
@@ -3183,17 +3183,19 @@ static const NSString* kStateCollectionB
   return [super contentRectForFrameRect:aRect];
 }
 
 - (NSRect)contentRectForFrameRect:(NSRect)aRect styleMask:(NSUInteger)aMask
 {
   if ([self drawsContentsIntoWindowFrame]) {
     return aRect;
   }
-  if ([super respondsToSelector:@selector(contentRectForFrameRect:styleMask:)]) {
+  // Call the instance method on super, if it exists (it's undocumented so we
+  // shouldn't rely on it), or fall back to the (documented) class method.
+  if ([NSWindow instancesRespondToSelector:@selector(contentRectForFrameRect:styleMask:)]) {
     return [super contentRectForFrameRect:aRect styleMask:aMask];
   } else {
     return [NSWindow contentRectForFrameRect:aRect styleMask:aMask];
   }
 }
 
 - (NSRect)frameRectForContentRect:(NSRect)aRect
 {
@@ -3203,17 +3205,19 @@ static const NSString* kStateCollectionB
   return [super frameRectForContentRect:aRect];
 }
 
 - (NSRect)frameRectForContentRect:(NSRect)aRect styleMask:(NSUInteger)aMask
 {
   if ([self drawsContentsIntoWindowFrame]) {
     return aRect;
   }
-  if ([super respondsToSelector:@selector(frameRectForContentRect:styleMask:)]) {
+  // Call the instance method on super, if it exists (it's undocumented so we
+  // shouldn't rely on it), or fall back to the (documented) class method.
+  if ([NSWindow instancesRespondToSelector:@selector(frameRectForContentRect:styleMask:)]) {
     return [super frameRectForContentRect:aRect styleMask:aMask];
   } else {
     return [NSWindow frameRectForContentRect:aRect styleMask:aMask];
   }
 }
 
 - (void)setContentView:(NSView*)aView
 {