When in 'course editing mode' the content of the course can cause a miscalculation of the width of the topic / week section. This is because width is set to 'auto' in the CSS and not '100%' to tell the browser to use all available space for the 'content' within its container. So in topics_collapsed.css:

#thetopics .section td.content, #thetopics col.content {
width: auto;
text-align: left;
}

needs to change to:

#thetopics .section td.content, #thetopics col.content {
width: 100%;
text-align: left;
}

And a similar change in weeks_collapsed.css.

Information on http://www.w3schools.com/cssref/pr_dim_width.asp.

Reported by Mike Stapleton of TeachRemote.com who kindly allowed access to his Moodle to solve the issue. Screen shot not available due to confidentiality.