Why does text butt up against the left edge of the editor?

I’m running Pinboard 1.1.8 on WordPress 4.0.1. For the sake of troubleshooting, I’ve disabled all plugins.

Issue summary: When Pinboard is the active theme, there is no margin or padding between the left edge of the post/page editor–the text is pushed up to the left border.

Steps to reproduce:

1. Set Pinboard as the active theme.
2. Go to the TinyMCE editor. You can:
   • create a new post
   • create a new page
   • edit an existing post
   • edit an existing page
3. If there is no text in the main editor, type some.
4. Observe the spacing on the left side of the editor.

Expected result: There is a small margin between the text and the left edge of the editor, as seen when Twenty Fourteen is the active theme:

Actual result: There is no such margin, and text butts against the left edge of the editor:

Other information: I use a Pinboard child theme usually (so I’m familiar with editing CSS). The process described above was tested with “vanilla” Pinboard, although I am also experiencing it in my child theme.

In particular, I’m pretty sure it has to do with this line in wp-content.css (line 20):
body.wp-autoresize { padding: 1px 0px !important; }

Why does the body need to have such minimal padding, and why does it need to be set to !important? It’s profoundly ugly and hinders readability, albeit in a minor way. I’d greatly appreciate some insight and a fix, if possible. I’m sure I could try to override that CSS with another, more specific !important rule, but I really don’t like warring !important’s.

Thanks!