Didier Verna writes: There are undocumented things that you will discover that code relies on. An example, VM had consistent display problems under Lucid emcas 19.10. I tracked the problem down to a set-window-buffer call. Under Lucid Emacs if the window already contained the buffer you were trying to attach with set-window-buffer it would just return. Under Emacs, it would cause w->force_start to be set. Because of a particular set of circumstances, the fact that Lucid Emacs didn't set w->force_start caused a newly created window to appear with point in the wrong place. I had to compare Emacs and Lucid Emacs code line by line to figure this out. The optimization Chuck added made logical sense, but there are so many factors that control where the cursor appears that he could not foresee this crazy new problem. See also this comment in window.c /* #### When the fuck does this happen? I'm so glad that history has completely documented the behavior of the scrolling functions under all circumstances. */ I had a heck of a time getting VM's scrolling code to work, and once I got it to work I was fearful of ever touching it because of the apparently voodoo involved in the redisplay. Witness this comment in VM: ;; This voodoo is required! For some ;; reason the 18.52 emacs display ;; doesn't immediately reflect the ;; clip region change that occurs ;; above without this mantra. It's been ten years since I added this code and I'm still afraid to take it out. The call to scroll-up that this comment referes to was needed because Emacs had to be coaxed to recompute _some_ of its display structures to notice a clippig region change. Another ugly exmaple: how about selective-display, selective-display-ellipsis and how they are supposed to work in the face of extents, faces and scrolling? I sure couldn't tell you how it is supposed to just by reading the documentation. The code is the only guide, such as it is. Maybe we would be better off if we wiped the slate and started over so we could be rid of this old crap. But we would have to be ready for some odd breakage. Toss selective-display and rewrite Outline mode to use invisible extents. Always return accurate information about what the display should look like, even if it requires much more on the fly computation. But we are indeed talking about a very different animal. I'm too chicken to try it myself, at least not all at once. I'm game to try to rewrite pieces of it. Pixel based scrolling could be added, I think, without starting over.