JavaScript dialogs cause a page to lose fullscreen.

chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.cc

Index: chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.cc
diff --git a/chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.cc b/chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.cc
index 3c129ddf63df97801ae902bdb359207226d48ff3..881ffe2029fde5f4cd846aae81aabbdce35f7aae 100644
--- a/chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.cc
+++ b/chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.cc
@@ -133,6 +133,11 @@ void JavaScriptDialogTabHelper::RunJavaScriptDialog(
       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) {
@@ -240,6 +245,10 @@ void JavaScriptDialogTabHelper::RunBeforeUnloadDialog(
   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));