owl/docs/playground/samples/window_manager/window_manager.xml

<templates>
  <div t-name="Window" class="window" t-att-style="style" t-on-click="updateZIndex" t-ref="root">
    <div class="header">
      <span t-on-mousedown="startDragAndDrop"><t t-esc="props.info.title"/></span>
      <span class="close" t-on-click.stop="close">×</span>
    </div>
    <t t-slot="default"/>
  </div>

  <div t-name="WindowContainer" class="window-manager">
    <Window t-foreach="windowService.getWindows()" t-as="w" t-key="w.id" info="w">
      <t t-component="w.Component"/>
    </Window>
  </div>

  <div t-name="Root" class="app">
    <WindowContainer/>
    <div class="menubar">
      <button t-on-click="() => this.addWindow('Hello')">Say Hello</button>
      <button t-on-click="() => this.addWindow('Counter')">Counter</button>
    </div>
  </div>

  <div t-name="HelloWorld">
    Some content here...
  </div>

  <div t-name="Counter" class="counter">
    <button t-on-click="inc">Click</button>
    <span><t t-esc="state.value"/></span>
  </div>
</templates>