Repro:

1. Open Demos -> Widget Gallery on the web demo
2. Run setInterval(() => console.log(document.activeElement), 1000) in a devtools console
3. Type something into the first text field
4. Click somewhere outside of the text field to blur it
5. Select the text by dragging over it - make sure the text field is blurred!

Expected behavior: runner.mutable_text_under_cursor is set to true, causing the <input> element to be the document.activeElement
Actual behavior: runner.mutable_text_under_cursor is false, and document.activeElement == document.body