The Inbox has a number of sub-folders.

Each folder has a number of email messages.

With the Inbox open, the user selects an email message.

Faster than the user can double-click to open the message, the Inbox folder is selected instead (the message turns "gray" instead of "blue").

This also happens with messages in sub-folders, Sent Items folder, and Deleted Items folder but at a much slower pace (the user can actually open or delete the message before it's unselected).

I have checked all her settings, Word is not her email editor and Outlook has been just fine up until yesterday afternoon. To our knowledge, no changes have been made to her computer.