When configuring, viewing, or editing a map service in Portal for ArcGIS 10.3.1, using Internet Explorer 11 on Windows 7 or 8, the 'Home' drop-down on the top ribbon can be clicked, but none of its child items can be selected (sub-list disappears). This behavior seems intermittent and may be related to the JavaScript button having incorrect sensitivity.
This could not be reproduced using Portal for ArcGIS 10.3.1 with IE11 (11.1540.10586.0 and 11.2248.14393.0). Since this is not reproducible on other systems, the issue may be system-specific and perhaps related to a specific combination of OS and IE11 version/settings.