On Mac, the Command (⌘) key prevents keyRelease events on regular keys.

Follow this scenario, while listening to QWidget's keyPressEvent and keyReleaseEvent:
1. Press down the command key
2. Press down a regular key (say 'T')
3. Release the 'T' key
4. Release the command key

Expected resulting sequence:
1. keyPressEvent (command key)
2. keyPressEvent (T key)
3. keyReleaseEvent (T key)
4. keyReleaseEvent (command key)

Actual sequence:
1. keyPressEvent (command key)
2. keyPressEvent (T key)
3. keyReleaseEvent (command key)

The attached minimal application shows the problem.
This is a problem when an application needs to track the set of pressed keys (not only modifier keys).