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Pretty sure he means the latter. Several editors I've used over the years supported that. The ones I can remember are Scite, UltraEdit, PNotepad and Vim.

I mean the latter (other open windows in the text editor). TextMate and Sublime both operate this way, as opposed to IntelliSense style auto-complete where it only operates off of symbol definitions (class names, function names, properties and variables, etc). Which makes sense, because TextMate and Sublime don't really "understand" your code like Visual Studio historically does.

Sublime Text doesn't do this by default although there's an extension[1] to do that.

[1] https://github.com/alienhard/SublimeAllAutocomplete

It auto-completes with words from tabs in the same VSCODE window. Atom has similar behaviour with autocomplete-plus.