Everything you press on in Pimcore will open as a separate tab:
If you wish to close all the tabs, then you can right click on one and choose "Close all tabs".
To get the best viewing possible, hide the tabs on right and left side when working.
When one tab is hidden, it looks like this:
If you press on the arrow in a tab, will that move from left to right tab- or from right side to left (as which side it was originally on)