Mouse Press/Drag behaviour on JToolBar is unlike Native Mercury toolBar.
Press mouse on any of the button on the toolbar(Do not release the mouse), then the border shown is Focus border(Dotted Border) for the native toolbar where as in JToolBar border shown is Roll over border.
Now without relasing the mouse move the mouse over other buttons, on Native the other buttons DO NOT show the roll over effect , but they show the rollover effect on JToolBar.

Note : For Native ToolBar Gedit is used.