onWindowKeyDown_(event) {
    // Add the key to the pressed keys set when it's pressed.
    this.pressedKeys_.add(event.key);

    const anySettingsMenusAreOpen = this.anySettingsMenusAreOpen();

    if (event.key == 'Tab') {
      // Enable blue outline for focused elements for keyboard
      // navigation.
      this.controlsContainer_.classList.add('shaka-keyboard-navigation');
      this.computeOpacity();
      this.eventManager_.listen(window, 'mousedown', () => this.onMouseDown_());
    }

    // If escape key was pressed, close any open settings menus.
    if (event.key == 'Escape') {
      this.hideSettingsMenusTimer_.tickNow();
    }

    if (anySettingsMenusAreOpen && this.pressedKeys_.has('Tab')) {
      // If Tab key or Shift+Tab keys are pressed when navigating through
      // an overflow settings menu, keep the focus to loop inside the
      // overflow menu.
      this.keepFocusInMenu_(event);
    }
  }
Add behaviors for keyboard navigation.
1. Add blue outline for focused elements.
2. Allow exiting overflow settings menus by pressing Esc key.
3. When navigating on overflow settings menu by pressing Tab
   key or Shift+Tab keys keep the focus inside overflow menu.

@param {!KeyboardEvent} event
@private