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Okay so after playing around with the html (since i already know what when wrong is just my html) i come up with this solution
<tbody v-for="(items,index) in groupData">
<tr class="active border-double" v-if="isGroup">
<td :colspan="thead.length"> // array for header, i just coun't how many header in the table to define colspan
{{index}}
</td>
</tr>
<tr v-for="item in items" :item="item">
<td>{{item.name}}</td>
</tr>
</tbody>
Source:stackexchange.com