An interesting feature has been added to the Gmail Labs: a useful preview panel, wich can be disposed as you want:
First go to the preferences in google mail account :??
click the gear on right side :
then in preferences, click on "Labs" (here "Labos because UI is in french"):
then choose the Preview Panel :
and activate it by clicking the "Activate" radio button.
And finaly , discover the new fetaure just added:
Thanks to Google labs 😉