In the editors I’m used to, the equivalent of “soft tabs” only causes tabs to insert 4 (or some other number) spaces, and backspace/delete to remove 4 spaces. It does nothing else.
Atom does more than that. Selection (whether by mouse or arrow keys) will treat the series of spaces as a unit; for example, hitting the left or right arrow keys once will move up or down a level of indentation, regardless of how many spaces that actually is.
While I can understand how it might be useful to have soft tabs be just as strict as hard tabs, I find it kind of disruptive. I click with the mouse, and my cursor doesn’t go where I expect it to. I use the arrow keys a couple of times, and suddenly I’m nowhere near the column I intended to be. Plus, the soft tab setting doesn’t affect old files, so it can be annoying to tell if a file is actually using soft or hard tabs.
How do I make Atom behave more like I’m used to?