Watch a video in fullscreen, e.g. on YouTube. Press Cmd + T to open a new tab.
Seemingly, nothing happens but once getting out of fullscreen you can see the tab (or multiple, if it was pressed numerous times) has opened but in the background.
Happens with compatability mode as well.
Fullscreen should be deactivated and the new tab should be opened in an "active" state.
That's how pretty much all other browsers do it, Safari included.
0.99.128.1-beta
Sonoma (14)