A list drill down on a report widget does not redirect users to the Service Portal UI, but to the standard UI.

This is the expected behavior because Service Portal lists don't support complex reporting query filters, only standard UI lists do.

1. Set up the redirection to the Service Portal for end-users as documented: Single sign-on, logins, and URL redirects
2. Install the plugin "Performance Analytics and Reporting - Service Portal Widgets"
3. Configure the report widget to display a report in a portal page as documented: Reports on Service Portal
4. Log in with a user that should be redirected to the portal.
5. Open the portal page displaying the report (ex. type "Bar").
6. Click on a data column in the report so to display a list of records.

Notice it navigates to a normal UI like /incident_list.do?sysparm... instead of /nav_to.do?uri=/incident_list.do that would use the portal UI as expected.

This is expected behavior and by design in the current releases.