There is a visual glitch when displaying the visual representation of Rule 4 for the GDD. The glitch happens the first time you open the visual representation and hover over the pop-up window with the mouse.
This is the window before hovering:

And this is the window after hovering:

I got the glitch with this file: r4-visual-bug.zip
To get it, prove that the lines CDG and EFG are perpendicular using GDD, and from the proof tab, click the green arrow to see detail, and then "RULE 4" to see the rule visual representation and description.