When elements are crammed together like tiles in a mosaic instead of overlapping like Windows 95, it's called a Tiling Window.
You might also see it called Non-blocking UI, or Non-overlapping UI, or Non-Modal Interface or Non-overlapping User Interface.
Tiling windows also tend to be in a tight relationship with the non-blocking philosophy which is slightly different. Non-blocking UI putsprograms put an emphasis on "not blocking" the user from making actions in any of the panels in any order. Sometimes in the case of a user error, you'll be blocked from proceeding until you make a correction, but in the most true non-blocking interfacessoftware, only the panel which encounters a block will fail while the others can continue to operate smoothly without that panel until the user gives the computer further direction. That's maybe more than you bargained for in an answer, but I wanted you to understand that it's not just a "look." It's a very old philosophy that has influenced user interfaces since the early days. It's also paired with initiatives in the programming world where certainCertain languages are very good at letting lots of interactions go at once while others get stuck if anything running in the main thread crashes. Without "asynchronous""asynchronous" programming paradigms, it wasn't feasible or reasonable to design an asynchronous noninput-blockingoutput user interface.