The bug
When filling slots in a chest, when in the last column hovering above it while dragging, the fist column slot under the slot you hover above turn darker. Works only with items having the enchanted glow for me. When you release the use key, it stops.
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look very closely at the bottom-left slot of the chest of the third screenshot. its slightly darker then the others
It seems like this only happens if one item at a time is dropped. Could also reproduce with items without the enchantment glint. It seems like the slot right of the selected slot is always darkened.
I just searched in MCP 9.31 (I know that there's a newer version, but I haven't installed it yet) for 1.10 and it seems like this bug can be fixed in the file RenderItem.java in the function func_184390_a(ItemStack, int, int, IBakedModel).
Just remove the line "GlStateManager.func_179140_f();" (forth from bottom to top), then this bug should be fixed. I'm not sure what side effects this has since I don't really know what this line does.
I'm sorry I don't see the issue