Option to continue session if browser / tab is closed

With xpack.security.sessionTimeout, you can extend the session timeout but it will always log out if the tab or browser window is closed. There are many systems (Slack, Skype, etc.) that connect via SSO that will keep the connectivity because the login credentials are kept by the system.

The original user making this request is hooking up Kibana/Elastic to SAML, and are finding that browser restarts always invalidate their sessions. This is as-designed per https://www.elastic.co/guide/en/kibana/current/security-settings-kb.html, but this user would like to have that behavior changed or changeable.