Check if dropdownlist is expanded or not.

Hi there, 
I'm currently doing a class project about a management system.
I've created an asp:DropDownList to display information of Employee.  <asp:DropDownList ID="employeeDropDownList" onkeypress="return KeyPressHandler(event)"></asp:DropDownList>

In KeyPressHandler(event), I check if user press Enter, the item is selected and form is submitted then.
When I run on FireFox, Chrome, .. as the DropDownList is expanded and I press Enter, the item is selected and I have to press Enter again to submit my form.
But on IE (every version), as the DropDownList is expanded and I press Enter, item is selected and the form is submitted immediately. All I want now is to make my DropDownList behaves as it does on Chrome, FireFox -> I have to press Enter 2 times as the DropDownList is expanded to select item and submit my form. Can anyone help me please ? Thank you.