The reason is that the old ExportAction is setting Content-Disposition incorrectly (which causes opening of JCR exports in browser window).

Even though this code is there:

        resource.getStream().setParameter("Content-Disposition", "attachment; filename=...");
        Page.getCurrent().open(resource, "", true);

The Content-Disposition header is set just to filename=... (without attachment;). (Probably a Vaadin issue - see https://vaadin.com/forum#!/thread/85013).

The JCR exports were disposed only because of (wrong) MIMEType application/octet-stream. After this was changed to application/xml, it behaves differently.