Because it's Compatibility View.
Adding <meta> tag in the head.
<!DOCTYPE html> <html> <head> <meta http-equiv='X-UA-Compatible' content='IE=Edge' />
Thanks for the tip. I ended up only using a portion of your suggestion, which was right on: "<!DOCTYPE html>" was the solution (mostly).
I started out by just adding that tag to the definition (there was no Doctype tag previously) and all the functionality was restored. There was still a visual anomaly, though: the placeholder text "Add new comment here" at the top of the widget did not correctly get removed when the user clicks into the field. Instead, the cursor shows up in the line above that text. And once you start typing a new entry, that new text shows up to the right of the placeholder text. Odd.
Adding the <meta> tag didn't seem to change anything, so I left that out.