Fullscreen Companion Window allows sites to place fullscreen content and a popup window on separate screens from a single user activation. This is a small requested enhancement of the Window Management feature: https://chromestatus.com/feature/5252960583942144
Sites have requested the ability to initiate compelling multi-screen experiences from a single user gesture, and Fullscreen Companion Window is an incremental web platform enhancement that fulfills their request. This feature builds on the Window Mangement API and the Tracking User Activation HTML standard. It allows sites with the window-management permission to open a single popup window when a fullscreen request is granted that targets a specific screen of a multi-screen device. Without this capability, users must make additional gestures or enable the broad "Pop-ups and redirects" content setting to initiate a compelling multi-screen content experience. See the design document for additional details.
Docs: https://docs.google.com/document/d/1RRlGQharWVnmxKTomfKhNiaeE31L7iXHeXVfifOvwJA
Samples: https://michaelwasserman.github.io/window-placement-demo
Explainers: https://github.com/w3c/window-management/blob/main/EXPLAINER_initiating_multi_screen_experiences.md