Text in text area moves one line below on the event of drop down selected index changed

See more:

C#

ASP.NET

Javascript

CSS

jQuery

Hello,

I have created a form using one drop down and one textarea. When i select value from drop down. the text in text area moves one line below. and The cursor in my textarea is not starting at the beginning when clicked to enter text on page load.
But if i remove auto postback="true" from drop down this problem is not occurring. so Let me know if any one know the solution for this Problem.

  <asp:DropDownList ID="ddlRegItemMainGroupCodeDesc" runat="server" CssClass="ddset" AutoPostBack="true" TabIndex="4" OnSelectedIndexChanged="ddlRegItemMainGroupCodeDesc_SelectedIndexChanged">
</asp:DropDownList>
   <asp:TextBox ID="txtRegSubGroupDescription" runat="server" class="txtfield2" TextMode="MultiLine" onKeyDown="textMaxCounter(this, 200, cphContent_RegSubGroupDescription)" Height="70px" Width="252px">
</asp:TextBox>

C#

protected void ddlRegItemMainGroupCodeDesc_SelectedIndexChanged(object sender, EventArgs e)
      {
          ddlItemMainGroupCodeDesc.SelectedValue = ddlRegItemMainGroupCodeDesc.SelectedValue;
      }