The selection drop-down list for Export Pages isn't working on half screen/browser, when the left/right panels are moved to the bottom of the page

Steps to reproduce:

1. Login as Admin
2. Go to Administer Wiki > Content > Export
3. Select 1 at a time all the 3 choices from the drop-down list under Pages
4. Resize the browser size - minimize it till the left/right panels move to the bottom of the page
5. Select 1 at a time all the 3 choices from the drop-down list under Pages

Expected results: The drop-down list is working and the user can make the required selections.

Actual results: The drop-down list isn't working when the page is on half screen, when the panels are moved at the bottom of the page.