Symptoms

After a Confluence upgrade, drop-down menus no longer collapse, despite moving the mouse away or clicking elsewhere. For example:

Causes

1. This is usually caused by upgrading the JDK (usually from 1.6 to 1.7), as old information can still be cached.
2. It can also be caused by a custom layout that is no longer compatible post-upgrade.

Resolution

1. Flush your plugin cache to clear any cached information from the previous JDK version.
   
2. Check your customisations via Confluence Admin > Layouts
   1. Start with the Global Layout and Page Layout. 
   2. If any are customised, copy the contents of them to a text file to save for later use
   3. Reset them to Default

   4. Regardless of whether this works or not, now is a good time to review your customised layouts, so compare your customised layout to the current default one, and re-enter any customisations that you require (that do not cause the problem to recur). 

      A note on layouts - if they are customised, they will not be changed on upgrade, so as not to destroy your customisations. This can occasionally mean that layouts break post-upgrade, so rather than simply copy/pasting them back in after these steps, it's better to extract your customisations from the old layout and re-apply them to the new default layout.