Intro

In #35117 I included changes to OsuGame which allowed the footer to be displayed in subscreens for use in the playlist song selection. The change is fragile as evidenced by the failing tests, so I set out to do this more properly.

This is the culmination of about 6 attempts at this, because the footer is extremely complicated. There are many edge cases to consider, such as:

• We still have a legacy back button, the state of which is controlled by both OsuScreen.BackButtonVisibility and whether the ScreenFooter itself is visible.
• Some screens selectively show or hide the footer.
• Some screens selectively show or hide the legacy back button.
• Some screens work with the footer in other ways, which I'm not sure how to handle more universally at this point.
• Footer buttons are expected to be single use, and disposed/expired when changing screens. In practice this means buttons must be recreated all the time.
• Footer buttons are expected to be cleared/disabled as soon as the screen is pushed or exited. In practice this means putting related code in OnEntering()/OnExiting() produces slightly different behaviour.

Therefore I've tried to keep the changes somewhat simple for this attempt but I still foresee this going through several refactoring efforts.

Outline

I've tried to imagine somewhat complicated scenarios here, and reduced it to tracking what I call the "leading screen" which is the most-nested 'current screen'. For example:

Parent -> Parent
          ^^^^^^

Parent -> [ Child -> Child ]
                     ^^^^^

Parent -> [ Child -> Child ] -> Parent
                                ^^^^^^

Parent -> [ Child -> [ Child -> Child ] ]
                                ^^^^^

This is done by the new class ScreenStackFooter which handles both the footer and the legacy back button almost exactly as OsuGame already does it, except with the above leading screen detail included.

Testing

Besides the new included tests, I've also tested this works with #35117.