Scenario:

• Having Documentation Theme as global theme
• Having a space with Documentation Theme set
• Another space with other theme for example Easy Reader Theme

If you navigate to a page within the space with Documentation Theme, sidebar is displayed on the left (if not, you can show it either by dragging the splitter or using the show/hide button on the top right corner). If you navigate to a page in the space with the Easy Reader Theme, then back to the space with Documentation Theme, the sidebar gets hidden.

Workaround: Set the global theme to the Default theme under Confluence Admin > Themes.