On the right-hand side (at the very top) in Post-processing you will find the 1:1 magnifying glass. The magnifying glass shows you the area the cursor points to in a 1:1 pixel view.
When clicking on the magnifying glass the new compare-window will appear, comparing your edited image and the original image. It simultaneously gives you the opportunity to examine the comparison through the magnifier display.
To keep the magnifying glass on a specific point, press down on the “L“-key while you move the cursor over the screen. To free the fixated area of the magnifying glass, press down on the “L”-key once more.
Click into the image area of the magnifying glass to change between the display-mode “divided before/after comparison” and “direct before/after comparison”.