I found this post by the developer here describing how Sublime GUI is drawn and managed by its own cross-platform UI toolkit.

Jon - I am very impressed by the performance of Sublime GUI toolkit you developed. Are you using some cross-platform OpenGL library to draw your window elements? Is it being done via OpenGL or something else - and if so, can you please share which cross-platform OpenGL base drawing library you are using? Also - the menus look very nice in Windows, I can’t tell that the menus are not native - how are you drawing the menus? Is it something like GLUI? What resources could you recommend towards an aspiring custom GUI developer? Thanks. Sublime Text is one of the few cross-platform GUI applications in which I don’t find any amount of “lagginess”.