PSA: Emacs is not a proper GTK application

29/10/2016

Update: I’ve been pointed to an emacs-devel discussion about giving Emacs a proper GTK frontend, most comparable to the Win32 and NS frontends (which are free from X11isms). This would be a better approach than what’s been outlined below, with the Cairo code being repurposed for the drawing bits.

Daniel Colascione’s excellent write-up on bringing double-buffered rendering to Emacs has prompted me to do the same on a set of questions that can be occasionally spotted on #emacs:

Which GUI build of Emacs shall I choose?
What’s the difference between the GTK build and the other builds of Emacs on Linux?
Does the GTK build run on Wayland?
Does the GTK build run on Broadway?
Why does Emacs not render as nicely as <insert more modern text editor>?

If you’ve ever programmed a GUI application with a modern/popular GUI toolkit, you’ll have noticed that while it gives you loads of desirable features, it forces you to structure your application around its idea of an event loop. In other words, your application is forced to react asynchronously to user events. Games are pretty much the only major kind of graphical application that can get away with doing their own event loop, but end up doing their own GUI instead.

Now, the issue with Emacs is that it does its own event loop, pretending that the frontend is a textual or graphical terminal. It’s pretty much the graphical equivalent of a REPL and at the time it only had a text terminal frontend, this way of doing things worked out fairly well. However, by the time users demanded having pretty GTK widgets in Emacs, it became clear that more involved hacks were needed to make that happen. This is why Emacs runs the GTK event loop one iteration at a time, pushes its own user events into it (to make widgets react) and a plethora of more hacks to reconcile their rendering with everything done by X11.

In other words, Emacs is more of a X11 application plus your favorite widgets. The choice of GUI toolkit to build it with is mostly irrelevant, save an infamous bug with the GTK3 frontend that can crash the daemon. Emacs will therefore not run on a pure Wayland system or under Broadway in the browser. If anyone would want to make that happen, either the GTK frontend would need to yank out everything X11 (unlikely as it’s deeply entrenched) or to create a new frontend doing platform-agnostic drawing (the Cairo feature being a prime candidate for that).