Scroll bars gone after cancelling the atto image dialog

Detected while playing with MDL-55809-

To reproduce:

• Go to any form with editor contents having an image, or create it. I've reproduced the problem consistently both in glossary entry edit and also user profile.
• Within the editor, click once on the image, so it's selected.
• Click in the "image" atto button, you'll see the standard image properties dialog, and the page behind it is properly "frozen" (no scrolls, dimmed...)
• Exit the dialog by clicking on the X (upper-right).

Expected:

• The page is "unfrozen", and can continue using it normally.

Existing problem:

• Scroll bars are gone and cannot continue with the form/page anymore.

Reproduced both with clean and boost under Moodle 3.2. Have not checked if the same happens with older versions neither if other dialogs lead to the same "unscrollable" behavior.

Ciao