Yeah, this seems like a bug with Riot.js/Split.js thing that pretty much shapes the entire Perchance interface. I also stumbled upon the bug long days ago when I started discovering Perchance.
But interestingly enough, when I maximize my browser window the bug sort of disappears. I did try the bug on an unmaximized window like above.
Also, there should be a way to make the tester panel appear even while the left side of the editor collapses (e.g. a floating tester panel, possibly draggable too).
Yeah I suspected something like this is happening. I don't think it's randomly adding a clearly wrong style though. I think it's probably wrapping.
One possibility is it's got float:right. But then if its width is not perfectly set to counter the width of whatever is on the left before it in the DOM, it will be pushed to the line below. Which it seems like is happening here.