The idea of specifying "bleed" is to make the carousel aware of its
vertical display area. The top bleed is under the filter control; bottom
beneath the toolbar. At the end of the day, the point of panel X offset
incursion, and the scroll target for current selection, should be at the
centre of the screen.
The fixes match code which already exists in the previous
implementation. Basically, without incorporating `BleedTop` into
calculations a second time, the centre position would not match
expectations (of being the centre including bleed).
This one probably doesn't matter as much because it's used as a single
instance in `TaikoPlayfield` (so its lifetime should end around the same
time as the `HealthProcessor`).
Beyond isolation, this stuff is also used to test (de-)serialisation.
Much-of-a-muchness in this case since it isn't too thoroughly tested,
but best do it anyway.