Requested in MDL-84891.

Steps to reproduce:

• Create a new course
  • Set the "Format" setting to "Custom Sections"
  •  Set the "Course layout" setting to "Show one section per page"
• Activate Edit mode
• Rename a section to "Section 1"
• Create a sub-section under "Section 1" and name it "Sub 1"
• Add an activity or resource of type "Text and media area" to "Sub 1"
  • With the "Title in course index" field set to "text 1"
  • With the "Text" field set to "some text"
• Deactivate Edit mode
• Click on any section other than "Sub 1"
  => the "some text" text should NOT be visible at this time
• Click on "text 1" in the course index
  => The site will redirect you to "Section 1" which is incorrect. It should redirect you to "Sub 1"