A bit less sensitive to underlying DOM structure.

packages/application-extension/src/index.tsx

@@ -538,7 +538,7 @@ function addCommands(app: JupyterLab, palette: ICommandPalette): void {
   // specific selector since we really only want tabs that are
   // in the main area, as opposed to those in sidebars, ipywidgets, etc.
   const tabSelector =
-    '#jp-main-dock-panel > .p-TabBar > .p-TabBar-content > .p-TabBar-tab';
+    '#jp-main-dock-panel .p-DockPanel-tabBar.jp-Activity .p-TabBar-tab';
 
   commands.addCommand(CommandIDs.close, {
     label: () => 'Close Tab',