DropDownCheckBoxes: Using control in c#

In my new c # project in visual studio i try to use the DropDownCheckBoxes control

My problems using the DropDownCheckBoxes control are two below :

1. Why the checkbox remains attached in drop down check box ?

2. Why when I confirm the value select in the DropDownCheckBoxes with the ok button does the web page not remain on the DropDownCheckBoxes and reposition itself at the beginning of the web page?

How to do resolve this ?

Please can you help me ?

My c# code below :

<asp:DropDownCheckBoxes ID="DropDownCheckBoxes1" runat="server" 
    UseSelectAllNode="true"
    Enabled="true" 
    UseButtons="true"
    CssClass="pure-u-23-24" 
    OnSelectedIndexChanged="DropDownCheckBoxes1_SelectedIndexChanged">
    <Style SelectBoxWidth="400"
        DropDownBoxBoxWidth="250"
        DropDownBoxBoxHeight="200"
        SelectBoxCssClass="CheckboxList"
        DropDownBoxCssClass="CheckboxList" />
    <Texts SelectBoxCaption="[ -------------- ]" />
    <Items>
        <asp:ListItem ...></asp:ListItem>
        <asp:ListItem ...></asp:ListItem>
        <asp:ListItem ...></asp:ListItem>
        <asp:ListItem ...></asp:ListItem>
    </Items>
</asp:DropDownCheckBoxes>

CSS

.CheckboxList {
    font-size: 10px;
    color: #333333;
}