CalendarExtender run out of position during time- click only

hi, 

my calendar run out of position such as appear at the corner of website during my 1st clicked on it. After tat , the calendar go back to his normal position when i clicked 2nd, 3rd and n-times. any idea;s ? is ther any coding or properties where i can set it exactly on the position no matter how may time i clicked it ?

my code :

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<asp:TextBox ID="txt_PostingDT" runat="server" Enabled="True" CssClass="text-input medium-input" ></asp:TextBox><asp:CalendarExtender ID="CalendarExtender2" runat="server" TargetControlID="txt_PostingDT"
 Format="dd-MM-yyyy" CssClass="CalendarStyle" FirstDayOfWeek="Monday"  ></asp:CalendarExtender>