QFileSystemWatcher doesn't emit directoryChanged signal when a file in the watched folder is modified. Steps to reproduce:

1. Create a QFileSystemWatcher to watch a certain folder. Connect to directoryChanged.
2. Create and close a new empty file in the folder. directoryChanged is emitted.
3. Modify the file (add text) with, say, a text editor. Save it and close. directoryChanged is not emitted, although it's supposed to be as per documentation. Explorer detects the change and displays it immediately.

P. S. fileChanged signal is emitted if you subscribe to that particular file, but subscribing to every file doesn't seem like a good idea for folders with a lot of files.
P. P. S. I haven't tested it on Linux or Mac yet.