https://backstage.forgerock.com/docs/idm/6.5/samples-guide/index.html#integrated-prep-openam. IDM samples guide section 29.1 makes a mention of AM CORS support issue. It provides a link to AM doc
For AM, see Preparing for Installation in the Access Management Installation Guide. Pay particular attention to the following section on Enabling CORS Support. To support communication between IDM and AM, you must enable cross-origin resource sharing (CORS). To support this sample, you must edit the web.xml file before deploying AM, as described in the following section of the AM Release Notes: Limitations.
Following the "Limitations" link, one needs to scroll down about 1.5 pages to see the descriptions on CORS issue.
The CORS issue is critical to fullstack implementation, but it is easy to miss and difficult to locate in the docs. The desired outcome is to quote the entire Using the Documented CORS Filter With IDM Integration Causes Errors issue in IDM samples guide.