The Atom code editor. It was good to have a mainstream alternative to VS Code, it's a pity it reached end-of-life.
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Indeed (and the names are a clue). More specifically, Electron came out of the ideas in Atom, not the other way around.
Atom Shell/Electron was from the very beginning something you could use separately from Atom as a framework for creating desktop apps using Chromium/Node.js.