You are in the Functionality Section!
You can see that you have two column: One left for Active Tabs and on right for Inactive Tabs.
To make inactive one active tab? So simple: drag and drop your selected tab from the left to the right.
The same to the left to make inactive to active!
Each tab can be edited anytime to change the tab function, the icon tab and the name tab.
Remember that in full screen mode, your preview app is always present on the right part of your screen!
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