just <folder> doesn't run default in `Justfile` about just

[runeimp]

Casing doesn't matter to Just. You can also have some_file-name.just.

When you execute Just it searched the current directory and those going towards the root directory looking for a Justfile of some sort. It does not check subdirectories to the current directory unless specifically told to do so. Like how you mentioned with foo/my-recipe. It worked there because you specified the directory as part of the 1st argument so Just checked if that directory existed then checked that directory for a Justfile to see if there was a my-recipe recipe to call.

[rocket-matt]

I have discovered it works if you use just foo/ (note the slash).

So, if this is intended functionality (I can see why it might be because there might be a recipe called foo), then disregard this issue.

However, perhaps the functionality of looking for foo/justfile when typing just foo if and only if there isn't a foo recipe in the root would be useful?

[runeimp]

Yes that is intended functionality.

I'm pretty sure this was brought up early in Just's development and was decided against. I like that sort of functionality myself, but it does lead to potential ambiguity and this tool is now used in lots (several thousand ?) of CI/CD setups so even more likely not to change at this point.