Thanks for your help.
I tried it but the width will change dynamically so I changed it in javascript on line 789. I added the following part of code to css property of element
minWidth: subW-(parseInt($sub.css('border-left-width')) || 0) - (parseInt($sub.css('border-right-width')) || 0)
The full line of code is
$sub.dataSM('scroll', {
step: 1,
// cache stuff for faster recalcs later
itemH: itemH,
subH: subH,
arrowDownH: this.getHeight($sub.dataSM('scroll-arrows').eq(1))
})
.bind(getEventsNS([
['mouseover', function(e) { self.menuScrollOver($sub, e); }],
['mouseout', function(e) { self.menuScrollOut($sub, e); }],
['mousewheel DOMMouseScroll', function(e) { self.menuScrollMousewheel($sub, e); }]
], eNS))
.dataSM('scroll-arrows').css({ top: 'auto', left: '0', marginLeft: x + (parseInt($sub.css('border-left-width')) || 0), width: subW - (parseInt($sub.css('border-left-width')) || 0) - (parseInt($sub.css('border-right-width')) || 0), minWidth: subW-(parseInt($sub.css('border-left-width')) || 0) - (parseInt($sub.css('border-right-width')) || 0) ,zIndex: $sub.css('z-index') })
.eq(horizontalParent && this.opts.bottomToTopSubMenus ? 0 : 1).show();
Thanks