Ajax Calendar Extender inside the Gridview Template not appearing completely towards the bottom of the Gridview.

Dear Friends,

I am using the Ajax calendar extender for my gridview. When grid view has few records, when i go towards the bottom records, on clicking the textbox, the calendar starts appearing smaller but calendar appears fully in the top records. How can i solve this. Should i do any changes in my css?

/* Ajax Calendar Style */

.ajax__calendar .ajax__calendar_container {

border:1px solid #646464;
background-color:#ffffff;
color:#000000;
}
.ajax__calendar .ajax__calendar_footer {
border-top:1px solid #f5f5f5;
}
.ajax__calendar .ajax__calendar_dayname {
border-bottom:1px solid #f5f5f5;
}
.ajax__calendar .ajax__calendar_day {
border:1px solid #ffffff;
}
.ajax__calendar .ajax__calendar_month {
border:1px solid #ffffff;
}
.ajax__calendar .ajax__calendar_year {
border:1px solid #ffffff;
}
.ajax__calendar .ajax__calendar_active .ajax__calendar_day {
background-color:#edf9ff;
border-color:#0066cc;
color:#0066cc;
}
.ajax__calendar .ajax__calendar_active .ajax__calendar_month {
background-color:#edf9ff;
border-color:#0066cc;
color:#0066cc;
}
.ajax__calendar .ajax__calendar_active .ajax__calendar_year {
background-color:#edf9ff;
border-color:#0066cc;
color:#0066cc;
}
.ajax__calendar .ajax__calendar_other .ajax__calendar_day {
background-color:#ffffff;
border-color:#ffffff;
color:#646464;
}
.ajax__calendar .ajax__calendar_other .ajax__calendar_year {
background-color:#ffffff;
border-color:#ffffff;
color:#646464;
}
.ajax__calendar .ajax__calendar_hover .ajax__calendar_day {
background-color:#edf9ff;
border-color:#daf2fc;
color:#0066cc;
}
.ajax__calendar .ajax__calendar_hover .ajax__calendar_month {
background-color:#edf9ff;
border-color:#daf2fc;
color:#0066cc;
}
.ajax__calendar .ajax__calendar_hover .ajax__calendar_year {
background-color:#edf9ff;
border-color:#daf2fc;
color:#0066cc;
}
.ajax__calendar .ajax__calendar_hover .ajax__calendar_title {
color:#0066cc;
}
.ajax__calendar .ajax__calendar_hover .ajax__calendar_today {
color:#0066cc;
}

Aspx Page:

<asp:TemplateField HeaderText="Planned Start">
<ItemTemplate>
<asp:TextBox ID="txtPlannedStart" runat="server" Text='<%# Eval("PlannedStart","{0:dd-MMM-yy}") %>' CssClass="txtbox" AutoPostBack="True" onfocus="showCalendarControl(this);"  Font-Size="Small" style="height: 16px" Width="105px"></asp:TextBox>
<asp:CalendarExtender ID="PlannedStart_date" runat="server" TargetControlID="txtPlannedStart" CssClass="ajax__calendar" Format="dd-MMM-yyyy" />
</ItemTemplate>
</asp:TemplateField>

Please help me..Thanks in advance.