I believe that @xiamenese is referring to the “fixed width editor” option in Preferences /Options. On Mac, this checkbox is found in the Preferences dialog, Appearance pane, Main Editor selected in the sidebar, Options tab. I assume it’s in a corresponding position in Windows’ Options dialog.
Anyway, it’s on by default in v. 3.x, but it wasn’t in v. 2.x Mac or v. 1.x Windows. If you ever changed it on your Mac, it was probably so long ago that it’s slipped your mind. I suggest looking at your options and turning this one off if it’s on. Does that help?