JavaScript dialogs cause a page to lose fullscreen.

chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.cc

 // Copyright 2016 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/ui/javascript_dialogs/javascript_dialog_tab_helper.h"
 
 #include "base/bind.h"
 #include "base/callback.h"
 #include "base/feature_list.h"
 #include "base/metrics/histogram_macros.h"
@@ skipping 115 matching lines @@
       UMA_HISTOGRAM_BOOLEAN("JSDialogs.IsForemost.Alert", foremost);
       break;
     case content::JAVASCRIPT_DIALOG_TYPE_CONFIRM:
       UMA_HISTOGRAM_BOOLEAN("JSDialogs.IsForemost.Confirm", foremost);
       break;
     case content::JAVASCRIPT_DIALOG_TYPE_PROMPT:
       UMA_HISTOGRAM_BOOLEAN("JSDialogs.IsForemost.Prompt", foremost);
       break;
   }
 
+  // For security reasons, when a page requests a JavaScript dialog it forfeits
+  // fullscreen. http://crbug.com/670135
+  parent_web_contents->GetDelegate()->ExitFullscreenModeForTab(
+      parent_web_contents);

[meacer, 2017/06/01 22:26:01]

Drive by: Does this exit window-fullscreen (as opposed to tab-fullscreen)? If
so, it might be problematic on Mac where maximizing Chrome causes it to enter
window-fullscreen mode (entered with F11 on other platforms)

[Avi (use Gerrit), 2017/06/01 22:49:33]

No, this is only exiting html5 fullscreen, which is the kind that the page
triggers itself. User-initiated fullscreen is completely different than this.
(The tangled nature of the fullscreen APIs makes me :( )

+
   if (IsEnabled()) {
     if (!IsWebContentsForemost(parent_web_contents) &&
         dialog_type == content::JAVASCRIPT_DIALOG_TYPE_PROMPT) {
       // Don't allow "prompt" dialogs to steal the user's focus. TODO(avi):
       // Eventually, for subsequent phases of http://bit.ly/project-oldspice,
       // turn off focus stealing for other dialog types.
       *did_suppress_message = true;
       alerting_web_contents->GetMainFrame()->AddMessageToConsole(
           content::CONSOLE_MESSAGE_LEVEL_WARNING,
           "A window.prompt() dialog generated by this page was suppressed "
@@ skipping 87 matching lines @@
   // - they are critical to the user not losing data
   // - they can be requested for tabs that are not foremost
   // - they can be requested for many tabs at the same time
   // and therefore auto-dismissal is inappropriate for them.
 
   SiteEngagementService* site_engagement_service = SiteEngagementService::Get(
       Profile::FromBrowserContext(web_contents->GetBrowserContext()));
   double engagement_score =
       site_engagement_service->GetScore(web_contents->GetLastCommittedURL());
 
+  // For security reasons, when a page requests a JavaScript dialog it forfeits
+  // fullscreen. http://crbug.com/670135
+  web_contents->GetDelegate()->ExitFullscreenModeForTab(web_contents);
+
   return AppModalDialogManager()->RunBeforeUnloadDialog(
       web_contents, is_reload,
       base::Bind(&SaveUnloadUmaStats, engagement_score, callback));
 }
 
 bool JavaScriptDialogTabHelper::HandleJavaScriptDialog(
     content::WebContents* web_contents,
     bool accept,
     const base::string16* prompt_override) {
   if (dialog_) {
@@ skipping 96 matching lines @@
   dialog_callback_.Run(success, user_input);
 
   ClearDialogInfo();
 }
 
 void JavaScriptDialogTabHelper::ClearDialogInfo() {
   dialog_.reset();
   dialog_callback_.Reset();
   BrowserList::RemoveObserver(this);
 }