Option for alwaysVisible to only apply when area is scrollable

[madeleineostoja]

Following on from #4, it would be great if we had an option for alwaysVisible to only show the scrollbar if the area is scrollable. I guess you wouldn't want to change the default behaviour of the existing prop since that would be a breaking change, but I imagine you could overload it.

Eg:

• alwaysVisible={true} current behaviour
• alwaysVisible="scrollable" only visible if area is scrollable (content height > 100% of viewport height)
• alwaysVisible="always" same as true

[daylilyfield]

@madeleineostoja thanks for your proposal. and i try to implement this feature in this weekend.