Until now, toolbox scroll areas would block input from arriving behind
them, even when no visible element was clicked.
In addition, clicking on a button inside a toolbox would still send a
`MouseDown` event to things behind it. Specifically, the editor's
`HitObjectComposer` would receive these events and also place objects
when the user does not expect them to be placed.
This fixes another regression that occurred due to `ScrollContainer`s no
longer blocking input theirselves.
