Hidden or for me unknow NetBeans Feature to show two files side by side

In NetBeans I'd sometimes compare the some code in the same file side by side.

To get the same file opened a second time, there is a clone-Item in the right-click-menu on the tab-Item.

To see it side by side, I dragged one around to fit it side by side, but the
right-click-menu has a "New Document Tab Group". This does it automatically and
has also a "Collapse Document Tab Group".