Factorio Forums

Inventory slots generally have a blue background if filtered, e.g. player's main inventory, cargo wagons etc.:


Ammo slots do not show this blue background. If the slot is empty, it shows a partially transparent icon indicating the filtered item, although depending on the specific icon it can be quite hard to distinguish this from the fully opaque one. But if there is an item stack in the slot, there is no way of knowing whether a slot is filtered or not, since middle-clicking on a filled slot provides no feedback. This is the case for vehicle ammo slots but also the player's personal ammo slots in the quickbar area.

(From left to right: filtered empty slot, filtered filled slot, unfiltered filled slot)

I guess with the player quickbar slots there is the additional complication of the gun slot and the green selected weapon background highlight. Gun slots can also be filtered (only discovered this just now) and have the same issues. I guess the green highlight would override the blue filter background as it's higher priority info.

Thanks for the report,

Similar to the other, I have fixed it for vehicles and spiders, but will leave the character/player slots for another time