Yes and no. E.g. Atom does its line layout and rendering in Javascript, which manipulates the DOM, and Electron then renders things using C++ code: http://blog.atom.io/2017/06/22/a-new-approach-to-text-render....

Emacs' display loop is tens of thousands of lines of C that directly calls the native text-rendering API: https://www.facebook.com/notes/daniel-colascione/buttery-smo....