Gmail has a panel for quick viewing of emails.

The Gmail team has introduced a new experimental “View Area” feature that allows you to simultaneously see both the selected letter and a list of other letters. The function should perfectly suit those with a larger screen resolution.

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


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By default, the letter becomes read in three seconds after its viewing. If you do not like it, then this can be changed in the settings on the “General” tab:



via Official Gmail Blog