Make scrollbars only visible when a Div is hovered over?

Question

I am trying to figure out how to have a scrollable div that only shows its scrollbars when Hovered.

Example is Google Image search, in the image below you can see

Answer 1

Answer by @Calvin Froedge is the shortest answer but have an issue also mentioned by @kizu. Due to inconsistent width of the div the div will flick on hover. To solve this issue add minus margin to the right on hover

#div { 
     overflow:hidden;
     height:whatever px; 
}
#div:hover { 
     overflow-y:scroll; 
     margin-right: -15px; // adjust according to scrollbar width  
}