De-select "Show selection rectangle while recording". When you stop recording and want to record a different area, click the dotted rectangle ("Select a desktop rectangle..." pops up when you hover your mouse over it) and the previously selected area will show up again on the screen and you can stretch/shrink/move it for your new requirement.
I made the mistake of leaving the rectangle showing when I was also recording another area of the screen with different software, and my precious "other recording" was very dim.