OK, debugging this more myself, I have determined that "xdiskusage <filename>" executes the du program with the -x option, which is documented to "skip directories on different file systems". The /media and /snap directories on my system contain subdirectories that are apparently mounted as different file systems, so that seems to explain the existing behavior.
OK, debugging this more myself, I have determined that "xdiskusage <filename>" executes the du program with the -x option, which is documented to "skip directories on different file systems". The /media and /snap directories on my system contain subdirectories that are apparently mounted as different file systems, so that seems to explain the existing behavior.
Sorry for the noise.