Outlook 2013 New email opens behind main window

In Outlook 2013, there are occasional cases where, when a new email is opened, it opens behind the main Outlook window.

There are various web postings about this, including advice about removing Outlook add-ins, and registry fixes.

Facing one of these recently, the following steps were taken:

• Uninstall all versions of Office from Programs and Features. 
• Delete Office entries in the Program Files folder. There were a number of bits of previous Office versions present.
• Install Office 2013 from scratch, and disable unnecessary add-ins

After doing the above, the issue became intermittent.  Sometimes a new email would open behind, other times correctly in front.  This seemed to be a matter of machine performance.  If a new email was opened immediately after changing the focus in the Inbox list, (and while the preview was still drawing), the new email opened behind.  If a new email was opened a couple of seconds or more after changing the focus, it opened in front.

In this particular case, Disk Defrag on the C: drive had not run to completion for a long time.  Fixing the disk and running Defrag then solved the issue completely.