<title></title> preview shows last deleted character(s) vs. empty string

I've been working through the HTML/CSS exercises with my daughters, and they are loving them--nice work! We noticed an issue with the preview for the document's title element, such that when you delete the existing title, it still shows the last letter of whatever was there vs. an empty string. My daughters couldn't understand why it was mismatched (e.g., showing a 'C' when there was nothing there in the code editor).

STR:
1. go to https://www.khanacademy.org/computing/computer-programming/html-css/intro-to-css/p/challenge-seasonal-ids
2. Move your cursor to the right of `Challenge: Seasonal ids`
3. Press delete/backspace and notice the preview Title updates as you type
4. Notice that when you delete everything (`<title>`) you still have a preview doc title of 'C'

If you simply highlight all of `Challenge: Seasonal ids` and press DEL it will show the full title.

Reproduce the issue at the linked Khan Academy challenge by deleting the title text character by character in the HTML/CSS editor. Start by tracing how editor changes update the preview document title, then verify that an empty produces an empty preview title rather than retaining the last character.

Written by the indexing model from the issue text.