The darkroom view is where you develop your images. The center panel contains the image currently being edited.
Middle-click on the center panel cycle between “fit to screen”, 1:1 and 2:1 zoom.
Alternatively you can zoom between 1:1 and “fit to screen” by scrolling with your mouse. Scroll while holding the Ctrl key to extend the zoom range to between 2:1 and 1:10.