ASP.NET cross post back happens instead of simple post back
<p>I have textbox, two buttons and other controls on my page. I am also using masterpage.</p>

<p>TextBox has OnTextChange event. I have javascript Calendar control that updates textbox and do PostBack with javascript. This in UpdatePanel.</p>

<p>I have two buttons. One of them is regular button that calls OnClick event. But second one shold cause Cross PostBack and opens new window (I need PostBack here, this is not just "opening window").</p>

<p>This looks like this:</p>

<pre><code>&lt;UpdatePanel&gt;
...
    &lt;asp:TextBox ID="txtStartDate" CssClass="rep_date" OnTextChanged="txtStartDate_TextChanged" runat="server"&gt;&lt;/asp:TextBox&gt;
    &lt;img id="calendarA" src="../images/calendar_background.png" style="width: 20px; height: 20px" border="0" alt="" /&gt;
 ...
&lt;/UpdatePanel&gt;

&lt;asp:Button runat="server" ID="btnSubmit" PostBackUrl="VisitationReportOutput.aspx" Text="Submit" OnClientClick="aspnetForm.target = '_blank'" CssClass="btn" /&gt;
&lt;asp:Button runat="server" ID="btnExport" Text="Export" OnClick="btnExport_Click" CssClass="btn" /&gt;
</code></pre>

<p>So far it looks very nice and everything works fine: 
btnSubmit calls Cross PostBack and opens new window, but btnExport doesn't cause cross PostBack, but simple PostBack. Also TextBox works fine - I am using calendar to update textbox, OnTextChange event occurs and does everything I need.</p>

<p>Problem is here:</p>

<p>1) First, I click on btnSubmit and it opens new window (Cross postback).</p>

<p>2) I close this window</p>

<p>3) Then I click on btnExport -> and here is problem, it opens new window (it does Cross Postback).
The same happens with TextBox. If I use it directly, everything is fine. But if I use btnSubmit first, close window, and then update textbox through calendar it opens new window (Cross PostBack happens).</p>
