Also note that the user will be confused when they delete the whole directory and observe 0 bytes get freed. (I guess a similar problem is also there even if you double count.)

The point is, the problem itself is-ill defined. There's no solution to that other than scrapping or redefining the problem itself. And it's hard to define the problem precisely for a non-technical user.

> Also note that the user will be confused when they delete the whole directory and observe 0 bytes get freed.

Note that's already the case when the user removes files that are opened by some process.

On Linux yeah. On Windows no.

On any non POSIX system.