"I could not understand why the major platforms (Windows, MacOS) did not." I expect that it's because in order to have correct proportional scrollbars, it requ

"I could not understand why the major platforms (Windows, MacOS) did not." I expect that it's because in order to have correct proportional scrollbars, it requires you to know the exact height of the entire scrolling area (or have decently good heuristics for them, which is not as much simpler a problem than you might think), which is an expensive thing to require of a scrollable area. The more content the system can handle, the more expensive that is. It's a big thing to ask of a heterogenous collection of layout code. Even today there's some scrollbars that aren't as proprotional as they seem. emacs, for instance, seems to be using some cheating heuristics rather than being precise. Under normal coding or text editing usage you might not notice, but if you fill a buffer with a lot of very differently-sized text you may find the experience of scrolling around is less slick than you might expect. Browser scrollbars can also behave strangely while the page is loading, as I'm sure everyone has experienced even if they haven't noticed. In that case it's perhaps more obvious why a smoothly-scrolling, totally-correct scrollbar is a surprisingly hard problem in the general case.