Previously, the selection box was only guaranteed to contain the actual
body of the slider itself, the control point nodes were allowed to exit
it. This lead to a lot of weird interactions with the selection box
controls (rotation/drag handles, also the buttons under/over it) as the
slider anchors could overlap with them.
To bypass this issue entirely just ensure that the selection box's size
does include the control point nodes at all times.
Same deal with this class. Fully qualifying the type names because this
has `#nullable disable` and makes use of `NotNull` which is also present
in the `System.Diagnostics.CodeAnalysis` namespace and AAAAAAARGH
NAMESPACE CONFLICTS.