Sroll bar doesn't feel native on Mac Yosemite

On natives mac apps, the scrollbar get thicker when hovered by the mouse pointer. But Qt apps doesn't respect this scheme. Not sure if it's a bug or a non supported mac tricky behavior.

On both screenshots, the mouse is hovering the scrollbar. On the first picture, Mac Terminal, the scroll bar is large, but with my Qt test application, the scroll bar isn't larger, only its background is, wich look bizarre.

PS: first bug report, sorry for any mistake I could have done