Fuel Fired Generator glitches when stackable-fuel source has a returned item

[Maverick2797]

This is not normally a problem (eg lava bucket is unstackable->only one at a time, coalblock is expected to be consumed) but using a stackable fuel source that returns an item (eg empty glass bottle) causes the formspec to bug out a bit and the "returned" item is not returned anywhere until the slot is empty where only a single "returned" item is actually returned.
Glitching: "Split stacks" toggle and (empty) fuel-inventory blinks.

.
Link to upstream issue: minetest-mods#543 (comment)

[gpcf]

I've seen the "splitting stacks" button glitch also on other machines, such as alloy furnaces.

[h-v-smacker]

Regular furnaces do the same BTW — several bottles of ethanol in, only one empty bottle left in the end in the fuel slot.

[h-v-smacker]

Looks like we need a whole new inventory slot for "waste" then. It's doable for generators, but what about fuel-powered furnaces and alloy furnaces? That, or make generators work a bit like self-contained injectors that throw waste out on their own, but then again — what to do with fuel-powered devices and LV tier generators?

Also the replacement system is ridiculous. The replacement is returned by
minetest.get_craft_result(input) → output, decremented_input
in output.replacements until the last item, when it's passed back in decremented_input.items.