Whenever I move my factory to another spot, remove a belt on the inside or outside, or upgrade a belt on the inside or outside to a faster/slower version, the content of the belt on the inside of the factory gets spilled on the factory floor. If the belt isn't full it just spills on the belt itself, not breaking anything unless the belt is supposed to have different items on each side of the belt. But the content might also spill onto another belt and block the progress of the factory.

Would it be possible to prevent the belt from spilling in general?

e.g. by keeping that input/output belt on the inside of the factory independently of whether there is a belt on the outside and not spilling the content if the belt gets upgraded/downgraded only. Or by implementing an active provider chest where spilled objects get stored at.

Thanks for bringing this to my attention. The belt spilling mechanic was actually something I implemented. The items were originally just voided.

I don't really like either of the solutions you provided for various reasons
Keeping input/output belts: The mod actually creates two linked belts per connection. One in the factory interior and one for the exterior. It would be possible to keep the interior belt but if the building is destroyed then the only option is to destroy the outside belt. Belt spilling would still be a problem in this case
Active provider chest: Not all factory buildings have roboports, and this would be awkward before the logistic network is researched.

I have an idea for an alternative solution that I think would fix your issue:
Just prevent the spilled items from landing on belts. Make them only land on empty tiles. This would be simple to implement and would stop the problem of items landing the wrong lanes.

You've been reporting a lot of edge case bugs/issues that I probably wouldn't have found for a while. Thanks a lot. Are you testing this mod in sandbox?

Honestly I just wanted to play around a bit with this mod on Freeplay when you released it. I already have some experience in terms of bugs and every now and then when I'm playing strange things keep happening (Can't enter the building, building has no energy, belt with two different kind of items did get mixed up, items spilled on the ground). They always trigger my interest so I try to reproduce the bug. Other than that I also try to break things somehow, just for fun.

I suggested the crate method because moving factories around is one of the many aspects of this mod and I wouldn't like to clear my factory every time I move it from one place to another. Early on you might ignore the crate or clear it every now and then and later on you might use a roboport to clear it for you.

By the way, we got those tiles on the very corner of the factory where characters can walk but items can't be dropped on, are those suitable for dropping spilled items? And what if they get ignored for too long and there's no space available to drop them? Also is it possible to prevent items from spilling at all if the belt only gets upgraded or downgraded but not removed?

Off-Topic: Just realized there's an invisible copper wire between factory circuit connectors and other connectors or power poles, which is capable of transmitting energy. The wire becomes visible if you place that factory circuit connector or the pole as blueprint. But that I think is engine related?

Off-Topic: Just realized there's an invisible copper wire between factory circuit connectors and other connectors or power poles, which is capable of transmitting energy. The wire becomes visible if you place that factory circuit connector or the pole as blueprint. But that I think is engine related?

Yes, this is engine related.
Trying to disconnect this "ghost wire" throws "Source entity is not electric pole or power switch: cannon disconnect copper wire."

Honestly I just wanted to play around a bit with this mod on Freeplay when you released it. I already have some experience in terms of bugs and every now and then when I'm playing strange things keep happening (Can't enter the building, building has no energy, belt with two different kind of items did get mixed up, items spilled on the ground). They always trigger my interest so I try to reproduce the bug. Other than that I also try to break things somehow, just for fun.

I suggested the crate method because moving factories around is one of the many aspects of this mod and I wouldn't like to clear my factory every time I move it from one place to another. Early on you might ignore the crate or clear it every now and then and later on you might use a roboport to clear it for you.

By the way, we got those tiles on the very corner of the factory where characters can walk but items can't be dropped on, are those suitable for dropping spilled items? And what if they get ignored for too long and there's no space available to drop them? Also is it possible to prevent items from spilling at all if the belt only gets upgraded or downgraded but not removed?

I ended up implementing a system where spilled belts would never spill onto other belts, and always look for the nearest empty tiles. The spilled items will also be automatically marked for deconstruction.

I'm still not convinced the crate system is the best solution. Players who don't know what the box is for will be confused when "random" items start appearing inside. The "empty tile spill solution" spawns items right next to the belt connection which helps explain where the items came from.

I understand, thank you for that solution.

Just another thing about spilling: Is it possible to avoid spilling at all if the belt gets e.g. just upgraded from yellow to red or from regular to underground?