I first tried to shake the coffee to see if I could make a mess, didn't work. When the mug is empty, the github logo becomes visible and you can click through to the github repo.

That's an illustration of the difficulties. I got the disk, video dvd and even drank the coffee but I initially thought the notebook was some sort of box for the video so didn't realise you could open it to read!

I think that's on purpose/by design, so that users are nudged into exploring. Things like these were pretty common in adventure games back in the days, which I probably spent too much time on playing...

How do you open the notebook?

I can drag it around, no problem. The cursor also changes to a pointy hand when over notebook's right side, but clicking doesn't do anything. Nor does click-and-hold-then-dragging.

Clicking on the right side (pointy hand) worked for me.