[Mac] Prevent constrained windows (sheets) from opening while instant preview...

chrome/browser/cocoa/constrained_window_mac.mm

 // Copyright (c) 2009 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/cocoa/constrained_window_mac.h"
 
 #import "chrome/browser/cocoa/browser_window_controller.h"
 #include "chrome/browser/tab_contents/tab_contents.h"
 #include "chrome/browser/tab_contents/tab_contents_view.h"
 #import "third_party/GTM/AppKit/GTMWindowSheetController.h"
@@ skipping 51 matching lines @@
 
 void ConstrainedWindowMac::ShowConstrainedWindow() {
   should_be_visible_ = true;
   // The TabContents only has a native window if it is currently visible. In
   // this case, open the sheet now. Else, Realize() will be called later, when
   // our tab becomes visible.
   NSWindow* browserWindow = owner_->view()->GetTopLevelNativeWindow();
   NSWindowController* controller = [browserWindow windowController];
   if (controller != nil) {
     DCHECK([controller isKindOfClass:[BrowserWindowController class]]);
-    Realize(static_cast<BrowserWindowController*>(controller));
+    BrowserWindowController* browser_controller =
+        static_cast<BrowserWindowController*>(controller);
+    if ([browser_controller canAttachConstrainedWindow])
+      Realize(browser_controller);
   }
 }
 
 void ConstrainedWindowMac::CloseConstrainedWindow() {
   // Note: controller_ can be `nil` here if the sheet was never realized. That's
   // ok.
   [controller_ removeConstrainedWindow:this];
   delegate_->DeleteDelegate();
   owner_->WillClose(this);
 
   delete this;
 }
 
 void ConstrainedWindowMac::Realize(BrowserWindowController* controller) {
   if (!should_be_visible_)
     return;
 
   if (controller_ != nil) {
     DCHECK(controller_ == controller);
     return;
   }
   DCHECK(controller != nil);
 
   // Remember the controller we're adding ourselves to, so that we can later
   // remove us from it.
   controller_ = controller;
   [controller_ attachConstrainedWindow:this];
   delegate_->set_sheet_open(true);
 }