Hopper with container above, eg. chest, furnace, etc. is able to collect dropped items.
- Place a chest on a hopper
- Add a water-stream at the level of the chest to direct items towards the chest
- When items are dropped into the water-stream, they will be collected by the hopper as they are pushed near the chest's hitbox.
A hopper with container above should not collect, or detect, dropped items. From the wiki,
A hopper with a container above it, such as a furnace, chest, or another hopper, cannot collect dropped items; for run-time efficiency, the hopper's code doesn't check for floating items.
The hopper is able to collect items due to items entering the smaller hitbox of the chest.
This bug can negatively affect item sorters, item pipes, etc., that rely on "capping" a hopper with a container to prevent dropped items from going where they're not supposed to.