Can't edit text in header

This one is driving me bonkers. I can’t seem to edit existing text in my headers. If I follow the instructions in the documentation below:

To edit the content within a text Bric, either hit the return key on your keyboard or give the text Bric a second click, this will cause the actual text content to become highlighted. This text Bric is now in edit mode and will accept input from your keyboard.

The text gets highlighted alright, but the only editing I can do is delete the entire text. No amount of left-clicking, pencil icon clicking, or menu selecting will give me the editing I-bar icon and let me modify the existing text. I’m sure I’ve done this before but can’t do it any longer. What am I doing wrong?