--- a/widget/cocoa/nsCocoaWindow.mm
+++ b/widget/cocoa/nsCocoaWindow.mm
@@ -3957,16 +3957,25 @@ TitlebarDrawCallback(void* aInfo, CGCont
 
   mIsContextMenu = false;
   return [super initWithContentRect:contentRect styleMask:styleMask
           backing:bufferingType defer:deferCreation];
 
   NS_OBJC_END_TRY_ABORT_BLOCK_NIL;
 }
 
+// Override the private API _backdropBleedAmount. This determines how much the
+// desktop wallpaper contributes to the vibrancy backdrop.
+// Return 0 in order to match what the system does for sheet windows and
+// _NSPopoverWindows.
+- (CGFloat)_backdropBleedAmount
+{
+  return 0.0;
+}
+
 - (BOOL)isContextMenu
 {
   return mIsContextMenu;
 }
 
 - (void)setIsContextMenu:(BOOL)flag
 {
   mIsContextMenu = flag;