This only covers constant rate rate adjust mods. Mods like wind up/wind
down will need a more complex implementation which we haven't really
planned yet.
Due to the use of bindable flow provided by `BeatmapDifficultyCache` in
this usage, the display would briefly flash to zero while difficulty
calculation was still running (as there is no way for a consumer of the
provided bindable to know whether the returned 0 is an actual 0 SR or a
"pending" calculation).
While I hope to fix this by making the bindable flow return nullable
values, I think this particular use case works better with non-bindable
flow so have switched across to that.
I tried restructuring the hierarchy to avoid needing this added property
(moving the hover layer out of the border container) but this leads to
some subpixel leakage outside the borders which looks even worse.
Closes#6915.