If you set the application/game to be fullscreen and in OBS game capture source set the mode to "Capture any fullscreen application", OBS should capture the current foreground window and its rectangular coordinates.
If the application/game creates its fullscreen mode with a bad set of rectangular coordinates, then that is the fault of the application/game itself. Go to the application developers website and tell them they have a fullscreen mode rectangle bug so they fix their problem.
Click on the game capture source and in the preview window, snap the rectangle to the upper left hand corner and drag the right lower corner to the preview window bottom right. That will force OBS to use that as the frame size. Then click the lock icon for the game capture source. If the frame required upscaling or downscaling that introduces more CPU processing to do that operation.
