4 ways to close an application

I knew these ways

1. Clicking on the upper-right button aka the X button
2. Alt + F4
3. Selecting  the Close command from the system menu (the upper-left icon on a window)

And I just found out a fourth way, after so many years of using Windows.
By double clicking the upper-left icon you execute the default command, which is the Close command.

Hooray!