In the '60s era of single-screen teletype computing, one of the factors that set the NLS apart was its use of a windowed interface. In a paper-based world, people worked by arranging and stacking multiple pages all the time, but it took real genius to translate that concept onto a computer screen.
The demo used television overlays to place live video of the participants next to the data, with separate panes for commands and information.
Further developed at Xerox PARC and elsewhere, windowing became the cornerstone of modern operating systems. For decades we've been able to run dozens of overlapping windows on our computers and switch graphically among them. Nowadays we can do it on our phones too.