An end result of #22674 is that `SkinBlueprintContainer`s are only ever
created by supplying a `SkinComponentsContainer` to them. However,
`SkinBlueprintContainer` still contained remnants of code that suggested
it was designed to handle cases where more than the drawable supplied to
it contained more than one `ISerialisableDrawableContainer`, or even
zero.
The zero path is totally dead right now (because every
`SkinComponentsContainer` is *by necessity* an
`ISerialisableDrawableContainer`), and the more-than-one path is dead
*for now* (and potentially forever?). Therefore, just hard-couple
`SkinBlueprintContainer` to receive a single target container.
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