OS: Mac - 64bit
The standard behavior of Mac apps when you press Command+m (⌘+m) is to minimize the front window to the Dock. See https://support.apple.com/en-us/102650#copy. In XnView MP, it does not. I hesitated to post this as a bug since it's clearly not a functional defect--I imagine it's a function of whatever cross-platform development framework you use. But it's not the expected behavior on a Mac, and it's not a new feature, so...
Effect: A standard Mac behavior doesn't do anything.
To reproduce:
1. Open the app on Mac.
2. Hit Command+m.
Actual behaviour (bug): Nothing happens.

Expected behaviour: The app should be minimized to the Dock.
By the way, a few software communities I work in have started adding a field to their bug report forms that says, "Has our software made you happy today?" Particularly since this is my first post in your forum, allow me to say that I have really enjoyed XnView for years. It's been so helpful, and I as an Open Source developer myself, I appreciate the amount of effort involved in supporting free software. So thanks!
