I used the following approach as workaround:
- Go to Settings >> Mail >> Layout >>
- Under the Reading pane options, select "At the bottom"
- Save and return back to email list window. Reading pane at the bottom half will appear.
- Drag the Reading Pane border marker (top-right) to reduce the size of the pane to minimum possible.
- Now the single click will not open any email. Whereas double click will open the email in new window.
Thanks! This seems to be the best workaround for the problem. Per you suggestion, I've turned the Reading Pane on for now, chosen to have it displayed in a strip at the bottom of the Outlook window, and the single clicks no longer open the email.
(FYI I had to choose this workaround today because Outlook forced me to switch to the new version just now, so I guess there's no going back to the old version anymore.)