If the zoom property is set on the body element for a preset zoom your window control does not seem to correctly take this zoom into account for the position and width/height of the window. You can easily recreate this in the following example application: https://stackblitz.com/edit/react-qmwfbf-fzmj98?file=app%2Fapp.tsx,index.html

Simply open the window using the button and then maximize the window in the title bar at the top. You can see that the width/height calculation is much too large and is not correctly maximized to the available space. You can also see this in the attached video.

A similar problem has already been reported here for popups https://github.com/telerik/kendo-react/issues/2320 and there is also a solution for this using the scale property in the PopupProps here:

<PopupPropsContext.Provider
      value={(props) => ({
        ...props,
        scale: 1.25,
      })}
    >

Please add exactly the same scale property for the WindowProps.