back again !...
Just re-visiting this issue.
On several pages on this test site, i have a collapsible panel, which, when expanded, pushes the content behind the fixed footer.
my 'quick & dirty' fix is to put some empty tags at the bottom of the content, which seems to stop the content from disappearing, but there has to be a 'proper' solution.
examples include..
contact.php
sitemap.html