diff --git a/widget/src/mac/nsMacWindow.cpp b/widget/src/mac/nsMacWindow.cpp
index bb11ed747a15..e3699443c134 100644
--- a/widget/src/mac/nsMacWindow.cpp
+++ b/widget/src/mac/nsMacWindow.cpp
@@ -193,11 +193,12 @@ nsMacWindow::nsMacWindow() : Inherited()
   , mAcceptsActivation(PR_TRUE)
   , mIsActive(PR_FALSE)
   , mZoomOnShow(PR_FALSE)
+  , mZooming(PR_FALSE)
+  , mResizeIsFromUs(PR_FALSE)
 #if !TARGET_CARBON
   , mPhantomScrollbar(nsnull)
   , mPhantomScrollbarData(nsnull)
 #endif
-  , mResizeIsFromUs(PR_FALSE)
 {
   mMacEventHandler.reset(new nsMacEventHandler(this));
   WIDGET_SET_CLASSNAME("nsMacWindow");  
@@ -1188,8 +1189,14 @@ NS_METHOD nsMacWindow::SetSizeMode(PRInt32 aMode)
   if (aMode == nsSizeMode_Minimized) // unlikely on the Mac
     return NS_ERROR_UNEXPECTED;
 
+  // resize during zoom may attempt to unzoom us. here's where we put a stop to that.
+  if (mZooming)
+    return NS_OK;
+
   // already done? it's bad to rezoom a window, so do nothing
   rv = nsBaseWidget::GetSizeMode(&currentMode);
+  if (currentMode == nsSizeMode_Normal && !mVisible && mZoomOnShow)
+    currentMode = nsSizeMode_Maximized;
   if (NS_SUCCEEDED(rv) && currentMode == aMode)
     return NS_OK;
 
@@ -1197,10 +1204,10 @@ NS_METHOD nsMacWindow::SetSizeMode(PRInt32 aMode)
     /* zooming on the Mac doesn't seem to work until the window is visible.
        the rest of the app is structured to zoom before the window is visible
        to avoid flashing. here's where we defeat that. */
-    if (aMode == nsSizeMode_Maximized)
-      mZoomOnShow = PR_TRUE;
+    mZoomOnShow = aMode == nsSizeMode_Maximized;
   } else {
     Rect macRect;
+    mZooming = PR_TRUE;
     rv = nsBaseWidget::SetSizeMode(aMode);
     if (NS_SUCCEEDED(rv)) {
       if (aMode == nsSizeMode_Maximized) {
@@ -1211,12 +1218,47 @@ NS_METHOD nsMacWindow::SetSizeMode(PRInt32 aMode)
       ::GetWindowPortBounds(mWindowPtr, &macRect);
       Resize(macRect.right - macRect.left, macRect.bottom - macRect.top, PR_FALSE);
     }
+    mZooming = PR_FALSE;
   }
 
   return rv;
 }
 
 
+// we're resizing. if we happen to be zoomed ("standard" state),
+// switch to the user state without changing the window size.
+void
+nsMacWindow::UserStateForResize()
+{
+  nsresult rv;
+  PRInt32  currentMode;
+
+  // but not if we're currently zooming
+  if (mZooming)
+    return;
+
+  // nothing to do if we're already in the user state
+  rv = nsBaseWidget::GetSizeMode(&currentMode);
+  if (NS_SUCCEEDED(rv) && currentMode == nsSizeMode_Normal)
+    return;
+
+  // we've just finished sizing, so make our current size our user state,
+  // and switch to the user state
+  StPortSetter portState(mWindowPtr);
+  StOriginSetter originState(mWindowPtr);
+  Rect bounds;
+
+  ::GetWindowPortBounds(mWindowPtr, &bounds);
+  ::LocalToGlobal((Point *)&bounds.top);
+  ::LocalToGlobal((Point *)&bounds.bottom);
+  ::SetWindowUserState(mWindowPtr, &bounds);
+  ::ZoomWindow(mWindowPtr, inZoomIn, false);
+
+  // finally, reset our internal state to match
+  nsBaseWidget::SetSizeMode(nsSizeMode_Normal);
+}
+
+
 //
 // CalculateAndSetZoomedSize
 //
diff --git a/widget/src/mac/nsMacWindow.h b/widget/src/mac/nsMacWindow.h
index f6d6028a86dd..ac67a1796a01 100644
--- a/widget/src/mac/nsMacWindow.h
+++ b/widget/src/mac/nsMacWindow.h
@@ -117,7 +117,9 @@ public:
     NS_IMETHOD            	GetScreenBounds(nsRect &aRect);
     virtual PRBool          OnPaint(nsPaintEvent &event);
 
-		NS_IMETHOD              SetTitle(const nsString& aTitle);
+    NS_IMETHOD              SetTitle(const nsString& aTitle);
+
+    void                    UserStateForResize();
 
   	// nsIKBStateControl interface
   	NS_IMETHOD ResetInputState();
@@ -148,16 +150,17 @@ protected:
                                                EventRef inEvent, void* userData ) ;
 #endif
 
-	PRBool							mWindowMadeHere; // true if we created the window
-	PRBool							mIsDialog;       // true if the window is a dialog
-	PRBool							mIsSheet;        // true if the window is a sheet (Mac OS X)
-	PRBool							mIgnoreDeactivate;  // true if this window has a (Mac OS X) sheet opening
-	auto_ptr<nsMacEventHandler>		mMacEventHandler;
+	PRPackedBool                    mWindowMadeHere; // true if we created the window
+	PRPackedBool                    mIsDialog;       // true if the window is a dialog
+	PRPackedBool                    mIsSheet;        // true if the window is a sheet (Mac OS X)
+	PRPackedBool                    mIgnoreDeactivate;  // true if this window has a (Mac OS X) sheet opening
+	PRPackedBool                    mAcceptsActivation;
+	PRPackedBool                    mIsActive;
+	PRPackedBool                    mZoomOnShow;
+	PRPackedBool                    mZooming;
+	PRPackedBool                    mResizeIsFromUs;    // we originated the resize, prevent infinite recursion
+	auto_ptr<nsMacEventHandler>     mMacEventHandler;
 	nsIWidget                      *mOffsetParent;
-	PRBool                          mAcceptsActivation;
-	PRBool                          mIsActive;
-	PRBool                          mZoomOnShow;
-	PRBool                          mResizeIsFromUs;    // we originated the resize, prevent infinite recursion
 	
 #if !TARGET_CARBON
 	ControlHandle      mPhantomScrollbar;  // a native scrollbar for the scrollwheel