> Maybe my Pythagorean cup rabbithole is still not over...

I'm sorry! ;) And I will love to see it printed and/or turned into another post.

On second thoughts, I still don't think it would work.

As you tilt the cup towards your lips, if the "highest" siphon pickup is still submerged, then that will correspond to the "lowest" siphon top and it will start siphoning anyway.

It would work as long as it could be arranged that the pickup would become exposed to air before the siphon started siphoning, but I'm not sure that's possible while still retaining the property that it self-starts when overfilled.

Also, even if the tube doesn't siphon because the pickup is exposed to air, there would still be liquid in the tube, some of which would get pushed over the top and leak out the bottom whenever you try to drink.

Connect them with a ring around the top. The high point will always be above the liquid level while drinking.

Nice, that would stop the siphon from self-starting!

Although it still wouldn't stop some water from trickling out the bottom when it tips over the "lowest" peak.