Thank you.
Sorry, I don't quite understand. If you put the cursor in the text box the control goes to the UI. If you click on the screen the control goes to the game viewport and you can use the mouse to rotate the camera.
I mean that in the original game, I think I remember having to hold down either the right or left mouse button in order to rotate the camera. It took some getting used to compared to most FPS games that, if they need to also use the cursor, have a button to enable the cursor and take control from the rotation rather than how you had it so the cursor was the default and the rotation was secondary. I hope that makes more sense.