grouped_view_widget.xml

<?xml version="1.0" encoding="utf-8"?>
<templates>
    <div t-name="account.GroupedListTemplate" owl="1" class="d-block">
        <table t-if="data.groups_vals.length" class="table table-sm o_list_table table table-sm table-hover table-striped o_list_table_grouped">
            <thead><tr>
                <t t-foreach="data.options.columns" t-as="col">
                    <th t-esc="col['label']" t-attf-class="{{col['class']}}"/>
                </t>
            </tr></thead>
            <t t-foreach="data.groups_vals" t-as="group_vals">
                <ListGroup group_vals="group_vals" options="data.options"/>
            </t>
        </table>
        <t t-if="data.options.discarded_number">
            <span><t t-esc="data.options.discarded_number"/> are not shown in the preview</span>
        </t>
    </div>

    <tbody t-name="account.GroupedItemsTemplate" owl="1">
        <tr style="background-color: #dee2e6;">
            <td t-attf-colspan="{{props.options.columns.length}}">
                <t t-esc="props.group_vals.group_name"/>
            </td>
        </tr>
        <t t-foreach="props.group_vals.items_vals" t-as="item_vals">
            <ListItem item_vals="item_vals[2]" options="props.options"/>
        </t>
    </tbody>

    <tr t-name="account.GroupedItemTemplate" owl="1">
        <t t-foreach="props.options.columns" t-as="col">
            <td t-esc="props.item_vals[col['field']]" t-attf-class="{{col['class']}}"/>
        </t>
    </tr>

</templates>