Functionally equivalent right now, but the combined variant is more
localised to what it actually needs to do, and less error-prone if
any new code gets appended to the method.
Reproduction scenario:
1. Hover a toolbox item
2. Unhover the item, but do not hover any other item (can be done by
exiting the toolbox completely to the right)
3. Come back to the item hovered in step (1)
4. The item would spontaneously contract after a second
It is impossible for the callback passed to `ApplyToBackground()` to
receive a null reference. See `OsuScreen.ApplyToBackground()` - if the
background to call the callback on were `null`, then an
`InvalidOperationException` would be thrown instead.