I just learned that from March 31, Bulgaria will join Schengen "by air and sea," but not by land, on March 31. Consider the following scenarios, all assuming a non-EU, non-Schengen citizen with visa-free access to Schengen and Bulgaria who has not been present in the Schengen area within the last 180 days:
From Istanbul, I drive to Sofia on April 1. On April 8, I drive from Sofia to Athens. On this itinerary, it appears that I enter Schengen on April 8, since Bulgaria is NOT part of Schengen by land, so I can stay in Greece for the full 90 days.
From Istanbul, I fly to Sofia on April 1. On April 8, I fly from Sofia to Athens. On this itinerary, it appears that I enter Schengen on April 1, since Bulgaria is part of Schengen by air, so I can only stay in Greece for 90 - 7 = 83 days.
From Istanbul, I fly to Sofia on April 1. On April 8, I drive from Sofia to Athens. On this itinerary, I appear to enter Schengen twice without leaving in between.
From Istanbul, I drive to Sofia on April 1. On April 8, I fly from Sofia to Athens. On this itinerary, I appear never to enter Schengen at all.
Now, I assume that I'm wrong about 3 and 4, since I shouldn't be able to enter Schengen twice in a row, and I shouldn't be able to be in Greece without entering Schengen. So what's actually supposed to happen on those itineraries?