Can't blockade monster above my own detection but within view of ally that sees the monster about freeorion HOT 2 OPEN

Saves are here https://freeorion-test.dedyn.io/FO0026-saves.7z

from freeorion.

I think what's happening is that blockadability of fleet / its ships is determined from stealth vs. detection, and not the empire visibility of objects (which can include other factors, such as allied shared visibility or effect-derived visibility overrides).

The reason that stealth vs. detection is used, rather than the empire's visibility of objects is that it is necessary to determine whether there is any fleet that could blockade a moving fleet even when the moving fleet doesn't start its movement in the system where the blockade might occur. So, the local visibility of a not-actually-there fleet potentially needs to be determined for all the systems along the fleet's route. Since the determination of whether a fleet can blockade another depends on whether the moving fleet can be detected, this can't just use the empire's start-of-turn visibility of the moving fleet, which was determined specifically for the fleet's start-of-turn location.

Doing a full visibility update for each potential location where a blockade might occur would be complicated, since it would depend on effects and visibilty sharing between empires, and might end up with weird ordering issues and inconsistencies if the result depends on whether other fleets have moved, etc.

So, instead, it just uses detection strength of the empire vs. stealth of the (ships of the) moving fleet. This is debatably plausible in terms of allied visibility sharing, as presumably there might be a communication delay for sharing that info.

It only seems weird in this case since the moving fleet is already visible, so there's an expectation that it should be equally visible for all considerations at the same location at the same time.

It could be reworked so that departing a system you start in uses the empire visibility and not the detection vs. stealth check, but that could potentially lead to some exploits / micromanagement quirks where if you start in a system, you can't leave it due to a blockade, but if you start at another system and just pass through the first system, you can pass due to the visibility calculations working out differently between the locations.

When deciding if a ship is a potential blockader at a system, I could perhaps do something like using the max detection strength of all the ship's owner and the owner's (allies who also have a ship in the system) instead of just the owner's detection strength. That would solve all the cases, though...

from freeorion.