Black Stained panes have wrong inventory texture

the stained glass appears to have a double outline on the top and left sides

You're looking at the inventory background texture.

It also becomes darker when it is placed in the inventory

Regular darkness when placed.

and lighter when it is being spread around and isn't yet placed

Slots you spread items across are highlighted, glass panes are partially transparent.

Thank you for clarifying that, it seriously looked like a texture glitch and I've never seen the highlighting upon inventory spread before.