It throws off my reading if I'm just checking that I know what something is and only need a quick skim to verify that. I immediately lost track of what was the explanation and what was the original text.
Can't speak for GP, but I experienced a similar moment of "ok now how do i close this again" before realizing I didn't really need to. But that didn't feel quite right either, like a popup that won't go away once its purpose has been served. Or like checking a footnote, and having a hard time finding my way back to wherever it was referenced. I get that what this library does isn't quite either of those things, but it feels enough like both of them that not being able to un-expound feels a little weird.