-[NSEvent buttonNumber] reports wrong values for mouse events on X11 backend

[doganov]

The documentation for -[NSEvent buttonNumber] clearly states that the left mouse button's number is 0, the right mouse button is 1, and so on: https://www.gnustep.org/resources/documentation/Developer/Gui/Reference/NSEvent.html#method$NSEvent-buttonNumber

Unfortunately, at least on the X11 backend, the buttons are numbered starting from 1. This becomes evident in the relevant source code, -[XGServerWindow _setupMouse], where generic.lMouse is initialized with buttons[0], which is Button1, which is 1.

libs-back/Source/x11/XGServerWindow.m

Line 1332 in 3b74bff

generic.lMouse = buttons[0];

Later, the value of generic.lMouse is used as is as a buttonNumber: argument when creating an NSEvent instance. This implementation leads to the left mouse button being reported by -[NSEvent buttonNumber] as 1, the right mouse button as 2, and so on.

I haven't tried the Wayland backend, but the comments in the relevant source file also doesn't inspire confidence that the documented semantics of buttonNumber is being followed.

[doganov]

The situation is actually worse than my initial report above. It is not only that on X11 the left mouse button is numbered as 1, but also the right mouse button is numbered 3, while the middle mouse button is numbered 2.

Expected:

• left = 0
• right = 1
• middle = 2

Actual:

• left = 1
• right = 3
• middle = 2

[DTW-Thalion]

Check out PR #160