[2.1] Appeal re-evaluate the prevention of fluid mixing

[Blaxor]

TL;DR

In 2.1, the new fluid mixing prevention blocks smoothie pipes for Thrusters and Electromagnetic Plants — machines that have bidirectional ports and use multiple fluids. I'd like this restriction to be reconsidered so that smoothie pipes remain available for these specific cases.

What?

Smoothie pipes are a known mechanic in Factorio that allow you to connect inputs or outputs of multiple machines handling different fluids into a single pipe. The game allows this as long as the connections are unidirectional.

However, as explained in FFF‑442, the rules change when connections become bidirectional:

"Notably, fluid mixing only occurs on bidirectional connections; you can still create smoothie pipes by connecting pumps or unidirectional machine outputs with different fluids in them, and the game will not get in your way."

The prevention was introduced to fix a specific bug with hidden double-buffers, not to remove smoothie pipes as a mechanic.

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The machines that have bidirectional ports are:
- Boilers
- Heat Exchangers
- Fusion Reactors
- Thrusters
- Electromagnetic Plants (for certain recipes)
For the first three (Boilers, Heat Exchangers, and Fusion Reactors), smoothie pipes are not particularly useful, as they only use a single fluid — unless you specifically want to connect a Heat Exchanger to a Fusion Reactor, which is a very niche case.
From this list, only Thrusters and Electromagnetic Plants actually use more than one fluid type. These are the machines we should focus on.

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Thrusters are the main focus among these machines. Using smoothie pipes with Thrusters simplifies pipe routing, making it more intuitive compared to traditional setups where each fluid requires a separate line. I don't believe this approach has any game-breaking consequences — it should be treated as a viable alternative rather than an exploit. The space savings are modest, and the design requires more thoughtful logic to manage correctly, which keeps it balanced.

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Electromagnetic Plants present a weaker case for smoothie pipes, as their port layout makes such setups less convenient compared to Thrusters. However, I believe it's still worth mentioning them: they have 10 recipes, several of which involve different fluid types. While the current port arrangement limits their smoothie-pipe potential, preserving the mechanic leaves room for future creative builds — someone might find a clever use for it if the option remains available.

Why?

I understand that the fluid mixing prevention was introduced to fix a specific technical issue — the hidden double-buffer in passthrough ports that allowed unrelated fluids to pass through, as described in FFF‑442. This was a necessary fix for a real bug.

However, as a side effect, it also blocks smoothie pipes for machines that legitimately use multiple fluids. Since the original problem was about the hidden buffer, not about intentional mixing, I believe this restriction deserves a second look.

The inconsistency makes it harder to understand the rules: smoothie pipes still work via pumps, so the mechanic itself is not considered harmful — it's simply selectively restricted. This feels arbitrary rather than intentional.

I'd kindly ask to reconsider whether Thrusters and Electromagnetic Plants could retain this capability — either through an explicit opt‑in toggle, or by treating their ports as unidirectional in the context of smoothie pipes.

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