If I have the webmail client opened with the Inbox being shown in a WIDE browser window, I see the list as normal... but as soon as I make the browser window NARROW, the list disappears and the first email will be opened. (The preview window setting is set to off but seems to be ignored in narrow windows.)
The width that is required to do this seems to vary based on what happened last. For example, if I simply viewed the Inbox, the width needs to be VERY NARROW to cause this. If I just purged marked as deleted, the width needs to be about what a mobile browser would be.
The issue with this is that on a mobile device, once I purge marked as deleted, the first email in the list gets opened instead of going back to the list after I purge.
I have employees/clients who are complaining to me about this.