WPF StackPanel Size -

i have following wpf window. why stackpanel not taking size of groupbox ? how can force auto take size of groupbox ?

thank you

xaml

  <window   xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"             xmlns:local="clr-namespace:styletrigger"             xmlns:local2="clr-namespace:comboboxdata"             xmlns:sys="clr-namespace:system;assembly=mscorlib"             xmlns:ig="http://schemas.infragistics.com/xaml" x:class="styletrigger.mainwindow"             mc:ignorable="d"             title="mainwindow" height="350" width="525">           <grid>             <grid.columndefinitions>                 <columndefinition width="50*" />                 <columndefinition width="50*"/>             </grid.columndefinitions>               <grid.rowdefinitions>                 <rowdefinition height="1*"/>                 <rowdefinition height="1*"/>                 <rowdefinition height="1*"/>             </grid.rowdefinitions>              <groupbox x:name="groupbox" header="groupbox" grid.row="1"  >                 <stackpanel  horizontalalignment="stretch" verticalalignment="stretch">                     <textbox />                     <textbox />                 </stackpanel>             </groupbox>          </grid>     </window>