Dropdown list (action menu) in Confluence 5.10 is visible in popup background

Steps to reproduce:

1. Create a space.
2. Open the space and open a page there, e.g. space homepage.
3. Click on the "..."-button (the action menu) in the upper right corner where you can e.g. export or delete a page.
4. Open a popup, e.g. "link to this page"

Expected result:
The dropdown closes automatically and the popup background is grayed out.

Actual result:
The dropdown is still visible in the background. It only closes if the user clicks into the popup.

Workaround:
Click into the popup and the dropdown will close.

Notes:
This also affects other dropdowns in the navigation section which could be created by third party addons.