I think the problem is that you do not understand very well the difference between
TabControl.ItemTemplate and
TabItem.HeaderTemplate .
Judging by the template you are trying to add a button to close the tab in the header of the header.
If so, then for this purpose use the following code:
<grid>
<grid.resources>
<datatemplate x:key="ClosableTabItemHeaderTemplate">
<datatemplate.resources>
<booleantovisibilityconverter x:key="B2V">
</booleantovisibilityconverter></datatemplate.resources>
<grid background="Transparent" horizontalalignment="Stretch" minwidth="100" tooltip="{Binding}">
<grid.columndefinitions>
<columndefinition>
<columndefinition>
</columndefinition></columndefinition></grid.columndefinitions>
<contentpresenter content="{Binding}" grid.column="0" horizontalalignment="Center">
</contentpresenter></grid></datatemplate></grid.resources></grid>
<datatemplate.triggers>
<trigger property="IsMouseOver" value="True">
<setter targetname="CLOSE" property="Visibility" value="Visible">
</setter></trigger>
</datatemplate.triggers>
<style targettype="TabItem">
<Setter Property="HeaderTemplate" Value="{StaticResource ClosableTabItemHeaderTemplate}" />
</style>
<tabcontrol>
<tabcontrol.items>
<tabitem header="This is test tab item with longer name." width="100">
<tabitem header="Short name" width="100">
</tabitem></tabitem></tabcontrol.items>
</tabcontrol>