@zacc I know you've already done some of these, but I was just thinking, rather than having to have `self.marionette.switch_to_frame(self.apps.displayed_app.frame)` everywhere we need to do it, why not add a method to the base object `switch_to_displayed_frame` that does it? In fact this was suggested in another PR [1] earlier.

It would move a bit of complexity out of each test's code, and also would encapsulate the behaviour in case it has to change at some point.

[1] https://github.com/mozilla-b2g/gaia/pull/13186#discussion_r7404558