JavaScript For Disabling mouse middle button on Dropdown

function Stop() { return false; }
var tmp = null;
function ondropdownfocus()
{
tmp = document.onmousewheel;
document.onmousewheel=Stop;
}
function ondropdownlostfocus()
{
document.onmousewheel=tmp;
}

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Call this function on onfocus & onblur of dropdown box in aspx.