At this point Marionette thinks that the keyboard button is visible when it is actually obscured by the "UI" toolbar button. so when it tries to tap the Keyboard button it ends up tapping the "API" button instead which is the topmost button!

Put this code in before the `tap`:
```

# Hack to make the keyboard button visible from underneath the toolbar
keyb_button = self.marionette.find_element(*self._keyboard_locator)
self.marionette.execute_script(
    'arguments[0].scrollIntoView(false);', [keyb_button])
```
Now the test will pass and you can merge :)