A trapped chest that has an upside-down slab or stair above it powers redstone dust adjacent to the block below it (including an Air block) when the chest is open.

The redstone dust is powered at level 1, the same as if it were adjacent to the chest iself. It can be on any side of the block below the chest. It can be directionless or configured to point in any direction. Only redstone dust is affected: If you replace it with a mechanism component, the component is not powered or activated.

Steps to reproduce

1. Place a trapped chest 1 block above the ground. (See screenshot 1)
2. Place an upside-down stair or slab above the chest.
3. Place a redstone dust adjacent to the air block below the chest.
4. Open the chest. Observe that the redstone dust has the particle effect indicating it is powered.
5. Place a repeater with its input side adjacent to the redstone dust. Observe that the repeater is powered by the dust, which shows that the dust is actually powered and not just affected by a rendering glitch.