Edit a page, and go to the very bottom. Type [[ so the link editor comes up. Enter a valid page, and hit Insert. Now when you type additional text, it is considered to be part of the link label, so the blue box expands.

My first workaround, which was very frustrating, was to hit Enter, type a space, then type the rest of my text, and then delete the hard newline. A better workaround that I only found while writing up this bug report is to hit the right arrow, which doesn't move the cursor, but does dismiss the dialog. Then I can enter text. Having that behavior for the right-arrow is counter-intuitive, and it's made worse by the odd visual feedback (that the cursor doesn't actually move).

I wouldn't expect it to matter, but I'm on Firefox under Ubuntu.