Email Script Editor not showing Standard Object tree

As title says.

When I try to add an email script token the script editor the tree on the right just contains a list of items labeled "Standard Objects" that keeps growing as I try to (unsuccessfully) add script tokens.

Here's what my editor looks like. Note the growing list of "Standard Objects" on the right, none of which are trees that allow me to drag tokens into the script editor window:

If I look in the Console (Chrome 67, latest) I see the following errors when I drag an email script token to the page:

I have no ad blockers installed in this profile.

Anyone else seen this or know what's causing it?