Image Browser does not return focus correctly

[Gold]

Using the TinyMCE image button opens a dialog to browse the images available to the user. If you click a filename to view the file, close the image view modal, and try scrolling the background page is what has focus and you can no longer scroll the image browser modal.

To replicate:
* Locate a TinyMCE editor with the insert image button.
* Click it to open the image browser modal
* Expand the 'Image' collapsible region
* Adjust the browser height so the modal can scroll
* Click an image filename to open the image view modal
* Close the image view modal
* Try to scroll the image browser modal

Expected result:

* Image browser modal scrolls

Actual result:

* Image browser modal is fixed in place and the underlying page scrolls

Version: 20.10 on https://demo.mahara.org/
OS: Ubuntu 18.04.5 LTS
database: Postgres on the demo site and MySQL on my local dev environment
Browser: Chromium 87.0.4280.66 (Official Build) Built on Ubuntu
Browser: Firefox 86.0 (64-bit) for Ubuntu canonical-1.0