Option to disable forcing `overflow-y: auto` for viewports

Currently, when an element is marked with `ui-scroll-viewport`, the `Viewport` constructor forces the element to have a `overflow-y` property with value `auto`. In the project I work on, we need the scrollbar to always show (`overflow-y: scroll`), but this override makes it very difficult to do so.

I'd like to request that `ui-scroll-viewport` be configurable to not override those relevant properties, or at least detect and not intervene if it's already `auto` or `scroll`.