On second thought, I think it would be easier to enforce the nonzero size in getViewport, then this change isn't needed and there's less ambiguity about baseLayer changes.

Yes, you can update getViewPort to make that adjustment.
Are there UA that require a certain minimum size for the viewport? If so, you could make the adjustment in that function as well.

On second thought, I think it would be easier to enforce the nonzero size in getViewport, then this change isn't needed and there's less ambiguity about baseLayer changes.

Yes, you can update getViewPort to make that adjustment.

I've moved the logic there.

Are there UA that require a certain minimum size for the viewport? If so, you could make the adjustment in that function as well.

I'm not aware of a specific UA requirement (beyond ensuring nonzero size), but I added an optional adjustment there where the UA could clamp to a minimum viewport scale.