Dropdowns on Special:ApiSandbox don't have overlay set, resulting in dropdown lists being limited to inner scrollable area.
This can probably be magically solved by passing { $overlay: $( 'body' ) } to constructors of OO.ui.DropdownWidget, OO.ui.CapsuleMultiSelectWidget and similar. This way the dropdown list dimensions will be limited by dimensions of viewport, rather than closest scrollable parent of the dropdown.