How to reproduce with Chrome :

• Go to tab "Users, groups and rights"
• clic on button "Right profiles"
• Let this panel opened
• then, select a page on the sitemap panel
• go to tab "page"
• clic on menu "Restrict access"
• let this panel opened
• then, try to add a page in the sitemap

-> 3 behavior possible :

• in the popup the content of the "Right profiles" panel appears
• or the button "add a page" does not work
• or the content on the popup appears in the "Right profiles" panel

See screenshot for more precisions