i3: Inconsistency in window frame offsetting

[Originally reported by lolilolicon@…] (I notice the window frame in i3 is handled inconsistently between the left frame and the top (title bar) frame. For example, in my case, the title bar is 20 pixels in height, so if I run mpv --geometry +0+20 video.mkv, the window frame is placed with its left-top corner at (0, 0); this is because i3 automatically shifts the mpv window 2 pixels (the size of the left frame/border) to the right, but does not automatically shift the mpv window down in the same manner. This results in unexpectedly that mpv --geometry +0+11 will make i3 put the mpv window at the center of the screen, presumably because i3 calculates that the left-top corner of the frame would be at (0, -9) and decides to resort to putting it at the center of the screen.

The discussion above ignores the fact that the win_gravity of the mpv window is actually CenterGravity, not NorthWestGravity, which I think may change the expected resulting frame coordinates. (To be tested in Mutter when I get the chance).