diff --git a/widget/src/cocoa/nsChildView.mm b/widget/src/cocoa/nsChildView.mm
index ac4b4685b42..c6405351d18 100644
--- a/widget/src/cocoa/nsChildView.mm
+++ b/widget/src/cocoa/nsChildView.mm
@@ -202,6 +202,9 @@ PRUint32 nsChildView::sLastInputEventCount = 0;
 
 + (NSEvent*)makeNewCocoaEventWithType:(NSEventType)type fromEvent:(NSEvent*)theEvent;
 
+- (BOOL)beginMaybeResetUnifiedToolbar:(nsIRegion*)aRegion context:(CGContextRef)aContext;
+- (void)endMaybeResetUnifiedToolbar:(BOOL)aReset;
+
 #if USE_CLICK_HOLD_CONTEXTMENU
  // called on a timer two seconds after a mouse down to see if we should display
  // a context menu (click-hold)
@@ -2504,6 +2507,42 @@ NSEvent* gLastDragMouseDownEvent = nil;
   NS_OBJC_END_TRY_ABORT_BLOCK;
 }
 
+// Unified toolbar height resetting
+// This fixes the following problem:
+// The window gets notified about the height of its unified toolbar when the
+// toolbar is drawn. But when the toolbar suddenly vanishes, it's not drawn,
+// and the window is never notified about its absence.
+// So we bracket drawing operations to the pixel strip under the title bar
+// with notifications to the window.
+static BOOL DrawingAtWindowTop(CGContextRef aContext)
+{
+  // Ignore all non-trivial transforms.
+  CGAffineTransform ctm = CGContextGetCTM(aContext);
+  if (ctm.a != 1.0f || ctm.b != 0.0f || ctm.c != 0.0f || ctm.d != -1.0f)
+    return NO;
+
+  // ctm.ty contains the vertical offset from the window's bottom edge.
+  return ctm.ty >= [[[[NSView focusView] window] contentView] bounds].size.height;
+}
+
+- (BOOL)beginMaybeResetUnifiedToolbar:(nsIRegion*)aRegion context:(CGContextRef)aContext
+{
+  if (![[self window] isKindOfClass:[ToolbarWindow class]] ||
+      !DrawingAtWindowTop(aContext) ||
+      !aRegion->ContainsRect(0, 0, (int)[self bounds].size.width, 1))
+    return NO;
+
+  [(ToolbarWindow*)[self window] beginMaybeResetUnifiedToolbar];
+  return YES;
+}
+
+- (void)endMaybeResetUnifiedToolbar:(BOOL)aReset
+{
+  if (aReset) {
+    [(ToolbarWindow*)[self window] endMaybeResetUnifiedToolbar];
+  }
+}
+
 // The display system has told us that a portion of our view is dirty. Tell
 // gecko to paint it
 - (void)drawRect:(NSRect)aRect
@@ -2601,6 +2640,8 @@ NSEvent* gLastDragMouseDownEvent = nil;
   paintEvent.rect = &fullRect;
   paintEvent.region = rgn;
 
+  BOOL resetUnifiedToolbar = [self beginMaybeResetUnifiedToolbar:rgn context:aContext];
+
   nsAutoRetainCocoaObject kungFuDeathGrip(self);
   PRBool painted = mGeckoChild->DispatchWindowEvent(paintEvent);
   if (!painted && [self isOpaque]) {
@@ -2610,6 +2651,9 @@ NSEvent* gLastDragMouseDownEvent = nil;
     CGContextFillRect(aContext, CGRectMake(aRect.origin.x, aRect.origin.y,
                                            aRect.size.width, aRect.size.height));
   }
+
+  [self endMaybeResetUnifiedToolbar:resetUnifiedToolbar];
+
   if (!mGeckoChild)
     return;
 
diff --git a/widget/src/cocoa/nsCocoaWindow.h b/widget/src/cocoa/nsCocoaWindow.h
index 1d6c603842e..fb83bb22f1a 100644
--- a/widget/src/cocoa/nsCocoaWindow.h
+++ b/widget/src/cocoa/nsCocoaWindow.h
@@ -172,12 +172,15 @@ struct UnifiedGradientInfo {
 {
   TitlebarAndBackgroundColor *mColor;
   float mUnifiedToolbarHeight;
+  BOOL mWaitingForUnifiedToolbarHeight;
   NSColor *mBackgroundColor;
 }
 // Pass nil here to get the default appearance.
 - (void)setTitlebarColor:(NSColor*)aColor forActiveWindow:(BOOL)aActive;
 - (void)setUnifiedToolbarHeight:(float)aToolbarHeight;
 - (float)unifiedToolbarHeight;
+- (void)beginMaybeResetUnifiedToolbar;
+- (void)endMaybeResetUnifiedToolbar;
 - (float)titlebarHeight;
 - (NSRect)titlebarRect;
 - (void)setTitlebarNeedsDisplayInRect:(NSRect)aRect sync:(BOOL)aSync;
diff --git a/widget/src/cocoa/nsCocoaWindow.mm b/widget/src/cocoa/nsCocoaWindow.mm
index acc05582525..d2f56efbe4b 100644
--- a/widget/src/cocoa/nsCocoaWindow.mm
+++ b/widget/src/cocoa/nsCocoaWindow.mm
@@ -1997,6 +1997,7 @@ static const NSString* kStateShowsToolbarButton = @"showsToolbarButton";
     mBackgroundColor = [NSColor whiteColor];
 
     mUnifiedToolbarHeight = 0.0f;
+    mWaitingForUnifiedToolbarHeight = NO;
 
     // setBottomCornerRounded: is a private API call, so we check to make sure
     // we respond to it just in case.
@@ -2047,6 +2048,7 @@ static const NSString* kStateShowsToolbarButton = @"showsToolbarButton";
 // unified gradient in the titlebar.
 - (void)setUnifiedToolbarHeight:(float)aToolbarHeight
 {
+  mWaitingForUnifiedToolbarHeight = NO;
   if (mUnifiedToolbarHeight == aToolbarHeight)
     return;
   mUnifiedToolbarHeight = aToolbarHeight;
@@ -2100,6 +2102,19 @@ static const NSString* kStateShowsToolbarButton = @"showsToolbarButton";
   return frameRect.size.height - [self contentRectForFrameRect:frameRect].size.height;
 }
 
+- (void)beginMaybeResetUnifiedToolbar
+{
+  mWaitingForUnifiedToolbarHeight = YES;
+}
+
+- (void)endMaybeResetUnifiedToolbar
+{
+  if (mWaitingForUnifiedToolbarHeight) {
+    // No toolbar was drawn, so set the height to zero.
+    [self setUnifiedToolbarHeight:0.0f];
+  }
+}
+
 - (void)setDrawsContentsIntoWindowFrame:(BOOL)aState
 {
   BOOL stateChanged = ([self drawsContentsIntoWindowFrame] != aState);