MDL-66979 behat: Fix window switching

Normalise switching of window contexts by setting the main window name
to a null.

Where the window has no name (null/empty), then the root context is
selected.

lib/tests/behat/behat_forms.php

@@ -67,9 +67,11 @@ public function press_button_and_switch_to_main_window($button) {
         // Ensures the button is present, before pressing.
         $buttonnode = $this->find_button($button);
         $buttonnode->press();
+        $this->wait_for_pending_js();
+        $this->look_for_exceptions();
 
         // Switch to main window.
-        $this->getSession()->switchToWindow(behat_general::MAIN_WINDOW_NAME);
+        $this->execute('behat_general::switch_to_the_main_window');
     }
 
     /**

lib/tests/behat/behat_general.php

@@ -203,13 +203,11 @@ public function switch_to_the_main_frame() {
      * @param string $windowname
      */
     public function switch_to_window($windowname) {
-        // In Behat, some browsers (e.g. Chrome) are unable to switch to a
-        // window without a name, and by default the main browser window does
-        // not have a name. To work-around this, when we switch away from an
-        // unnamed window (presumably the main window) to some other named
-        // window, then we first set the main window name to a conventional
-        // value that we can later use this name to switch back.
-        $this->execute_script('if (window.name == "") window.name = "' . self::MAIN_WINDOW_NAME . '"');
+        if ($windowname === self::MAIN_WINDOW_NAME) {
+            // When switching to the main window normalise the window name to null.
+            // This is normalised further in the Mink driver to the root window ID.
+            $windowname = null;
+        }
 
         $this->getSession()->switchToWindow($windowname);
     }
@@ -220,7 +218,7 @@ public function switch_to_window($windowname) {
      * @Given /^I switch to the main window$/
      */
     public function switch_to_the_main_window() {
-        $this->getSession()->switchToWindow(self::MAIN_WINDOW_NAME);
+        $this->switch_to_window(self::MAIN_WINDOW_NAME);
     }
 
     /**