I've discovered, since upgrading to a newer version of Notepad++, that I
sometimes press space while holding down shift when typing rapidly.
Since this is usually innocuous, I've never noticed before.

I have no idea what this feature is supposed to do, but if I rapidly
type something like "__ " before "sprintf," I end up with "__sprintf
sprintf."  This happens frequently and is both surprising and annoying.

Since I'm worried correcting this style will impact my typing speed, is
there some way to turn this Alt-Space feature off?

Separately, as a user and programmer, it seems that overloading
typewriter keys that didn't traditionally have a special meaning is a
mistake.  Windows Sticky Keys is a good example, having caused far more
annoyance than utility.