When i load a DC game with Reicast, the monitor behaves as if it’s 4:3 native. That means the black borders left and right cannot be used at all. You can’t even place an overlay there, it just appears inside the 4:3 window.
No options fixes this. Even if i manually choose a 16:9 ratio i only get a letterbox screen with added borders on top and bottom because the left and right borders can’t be filled.
This doesn’t happen with Naomi though. The same core and the same settings are fine when i load a Naomi rom. I can use an overlay normally there to fill the sides of the screen.