[Backport to release/4.0] - Bottom part of some sites is not shown

The bottom part of some sites is not shown in the channel manager. Whether that happens depends on the styling and markup of the site.

Steps to reproduce:
1. start the Selections Plugin demo
2. login to the CMS
3. view the 'Selection Demo' channel

Expected: the page is shown fully, as seen on localhost:8080/site
Actual: the last row in the table is cutoff (see screenshot)

The effect is more noticeable on the 'multilingual' page (see screenshots).