Gmail has a panel for quickly viewing emails

The Gmail team introduced a new experimental feature, “View Area,” which allows you to simultaneously see both the selected message and a list of other letters. The function should work perfectly for those with a higher screen resolution.

After enabling the "Viewing Area" on  the "Laboratory" tab in the mail settings , you will see a switch in the upper right corner of the message list, which will allow you to switch between three modes of viewing mail:



By default, the letter becomes read three seconds after viewing it. If you don’t like it, then you can change this in the settings on the General tab:



via Official Gmail Blog