Unlike glass, stained glass shows not only the sides of the block facing the player, but the other sides, too.
It is unclear whether this is a feature of stained glass or a bug.
Also, when against glass, the glass shows texture z-fighting.
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Not a bug. This is a feature, or more of an absence of backface culling.
I think normal glass should be like this, too.

Actually a dublicate of MC-34649.

@Kumasasa / Tails:
Anyone please resolve this?
also shows z fighting when next to a slab or stairs whereas it doesn't with normal blocks

Still to be resolved as a dublicate of MC-34649 !
yup, can a mod please close this issue?