Doors and trapdoors have a convenient grid of four 'windows' in them. Despite that these things are 3 'pixels' thick, looking at the outside edge of them, there is no 3-pixel-thick border to the windows, when you look through them.
This might be intended for better visibility, now that I think about it, but I thought I'd bring it up, since it seemed like I shouldn't be able to see straight through a trapdoor window, to see the cobblestone that it's attached to.
It immediately reminded me of MC-102 and MC-3908, but on the other hand, you don't have to put a block next to it for it to occur; it occurs pretty much no matter what is next to the door / trapdoor.
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Duplicate of MC-2333 , please use the search function to see if your bug has already been submitted. Currently over 30% of tickets are being closed as duplicate.

Thank you, Tails. I did look search, but only for trapdoors, so I missed this one.
Its probably like this because the renderer would take more ressources and on older computers, Minecraft would lag even more. Its not a bug so, but a feature request. And then you could say, that the renderer should be toggable for this with the "Fancy" graphics.