Steps to reproduce:

Using IE

1. Create a Space in Confluence with a single page. The Show/Hide button at the top right of the page works as expected, toggling sidebar visibility.
2. Add more pages to the same space (i.e. 100 pages under the space home page all on the same level.)
3. Show/Hide button stops to work, staying locked and collapsed.

WORKAROUND: Manually drag the Sidebar back to its visible position.