Will this change my existing combinators?
No. Existing combinators keep their current behaviour (the first dropdown option is always the unmodded one). The new modes are opt-in per combinator.

Do I need to unlock or craft anything?
No. There are no new items, recipes or entities. The modes appear in the normal combinator window.

Is it multiplayer-safe?
Yes. All modes are deterministic and run identically on every client.

Do the new modes survive blueprints, copy-paste and undo?
Yes. A combinator's mode and settings travel with blueprints, blueprint books, copy-paste, cloning and undo/redo. Vanilla-mode combinators carry no extra data.

Does Stack Pack handle fluids?
No. Slots are item slots, so only item signals are packed. Fluids, recipe signals and virtual signals on the same wire are ignored.

In Stack Pack, what happens when the slot budget runs out partway through an item?
That item outputs a partial amount — the part of it that fits in the remaining slots — so the budget is respected exactly rather than rounded to whole items. The partial stack fills first.

If I read the Stack Pack budget X from a signal, does that signal get packed too?
No. The signal you choose as the budget source is excluded from packing, so your control signal never eats a slot or leaks into the output.

Why are the combinator windows slightly different from vanilla?
Factorio hardcodes the vanilla combinator GUIs — a mod can't add options to them. To present the new modes as native dropdown entries, the mod recreates those windows. It tracks the vanilla layout, but base-game GUI changes may take a moment to catch up.

Does it work with other mods?
It works alongside most mods. Other mods that also replace these specific combinator windows can conflict, since only one replacement can win.

Does it support quality?
Yes. Quality is respected and carried through wherever it applies — recipe products and packed items keep their input quality, and same-item-different-quality signals are treated separately.