Wonderful, that does the trick!
I never use that ruler either, so there’s no big loss for me.
And for those who want to keep the ruler visible, the workaround is to select all text, then move both the left margin and the first-line indent to 0.5 cm.
If you do that, the horizontal ruler is still shown, but the text isn’t cut off anymore on the left side.
(Now why didn’t I think of that earlier?!)
While fiddling around a bit more, I discovered that the above (changing margin & indent) is in fact overkill, because showing and hiding the ruler is totally quick and easy, if you can remember the shortcut: Cmd-R.
To clearly define the situation: This is about TextEdit Rich text documents in ‘Wrap to Window’ mode, with System Settings » Appearance » Show scroll bars set to: Always.