The UI action cannot access the DOM objects and there will be a console error when it tries to access it. For example, assume a UI action is trying to set size for a UI page(dialog.setSize(window.innerWidth * 0.50, window.innerHeight);). There is will the following error on click the UI action.

Uncaught TypeError: Cannot read property 'innerWidth' of null

There is an 'Isolate script' checkbox in the UI action. When the 'Isolate script' is true, it cannot access window objects and 'innerwidth/innerheight' will be undefined.