Hi Peter,
If you want to have closing buttons inside the tabs(multiple) which are docked in one tab you can use the ‘tabCloseButtons’ property of the docking layout like in the following code snippet:
//In your Js file:

window.onload = function () {
    const docking = document.getElementById('docking-layout')
    docking.layout = [
            //The Base Group
            type: 'LayoutGroup',
            orientation: 'horizontal',
            items: [{
                    label: 'TabsWindow B',
                    size: '25%',
                    items: [{
                            label: 'Tab B1',
                            selected: true,
                            content: 'Content of B1'
                            label: 'Tab B2',
                            content: 'Content of B2'
                            label: 'Tab C1',
                            content: 'Content of C1'
                        tabCloseButtons: true,

And in your HTML file :
<smart-docking-layout id="docking-layout"></smart-docking-layout>
Please, do not hesitate to contact us if you have any additional questions.
Best regards,
Yavor Dashev
Smart UI Team