Managing your default reading pane preference
Open your profile from the top right of the screen using the profile icon with your initials 
. Select 'Mailboxes' from the drop down menu.
Select 'Tabs' from the email dropdown menu:
You have two features to choose from.
'Keep conversations open when switching tabs'
Enabling the option to keep conversations open when switching tabs ensures that the last opened email in each tab, stays open even when switching tabs.
Disabling this option will result in the most recently opened email being displayed across all tabs, rather than individual emails for each tab.'Automatically open email when browsing with keyboard'
Use keyboard arrow keys to scroll and open emails.