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If you need to set your course to use the Collapsed Weeks or Collapsed Topics course format, here's how:
1. "Edit Settings" from the gear icon (top-right of your course), and open the section called "Course Format".
2. In the "Format" list, select "Collapsed Topics" (if you need Collapsed Weeks, don't worry - keep reading...)
3. The page will automatically refresh when you make that selection. Look farther down the page for an option called "Set structure".
If you want Collapsed Topics, select "Topic" here; for Collapsed Weeks, select "Week" here.
4. Click "Save Changes" at the bottom of the page, and have a look at your collapsable course format.
(Experiment with the other colour, column and layout options if you want to - there are a lot of new Course Format options available as of Moodle version 2.5.)
NOTES:
You may find that the text colour is too light or hard to read in the horizontal collapsible title bars. The colour of the text may be set to a medium grey, even if a different colour is defined in the Collapsed Topics settings. This is because the overall theme we have selected for Moodle 2.5 is overriding the text colours set by the collapsed course format. You may improve legibility of title bar text by making the "Toggle background" and "Toggle background hover" colours darker.
We have observed that some courses which had previously been using the "Collapsed Weeks" course format under Moodle 2.2.4 had been set to regular (non-collapsing) Topics format after the system was upgraded to Moodle version 2.5. So, if your course uses a Collapsed course format, you may find that you have to set this again in your course shell.
More information from Moodle.org:
https://docs.moodle.org/en/Collapsed_Topics_course_format
Moodle Course Formats: Options and Settings: