Comment 6 for bug 726978

As mathbr said, this isn't something that's trivial to do in GTK+. Additionally, the space key is used by GTK+ to activate widgets by default, so users who are used to that function might be confused if we start overriding this behavior. Furthermore, since text entries are part the the UI and need to be able to absorb spaces for their own functionality to work, it wouldn't be guaranteed to pause when you switch back anyway! The action of space is ALWAYS going to be context-sensitive to one degree or another.

I do agree that it's a usability issue, and we should at least handle this for the common case where the focus is in the playlist. I'm not certain that it makes sense or is feasible to do it everywhere else, which is why I marked this as Opinion.