The Desktop GUI was perfected with NeXTStep in 1990. Though some might say Smalltalk in 1984 (modulo some nice artwork and colours).

We could even have unified desktop.mobile codebases, with different GUI layouts. 280 North's objective-J/Cappuccino proved this.

IDK what we're even doing anymore.

The early Mac made huge functional improvements over the Smalltalk GUI, improvements still used today.