1995 text editors didn't use a "layout engine" that executes JavaScript, attempting to JIT fragments of code. They were an event loop that processes native OS events and responds with repainting areas of the window. They also weren't able to automatically recognise language syntax and didn't have Git integration :-)
One of my favourite editors from the 1990's was FrexxEd, co-authored by the author of Curl, whose main event loop processed Arexx commands and events for every internal command, so you could rebind every event to a script in their own scripting language (FPL; C-like), and access every internal function from it. (It incidentally also came with FPL scripts that provided some degree of syntax highlighting for a few languages, and you could add more, though not as expansive as most modern editors).
Running a heavily scriptable editor with a GUI was entirely viable on a 7.16MHz Amiga with 1MB RAM, and more responsive than many modern editors.
Integration with other tooling, like RCS, compilers, or linkers was a given for Amiga apps at that time.
(FrexxEd was also notable for exposing internal buffers as files in a special filesystem, so you could e.g. lint or compile or call your revision control - limited as they were - directly on the buffers without any custom integration)
CSS grid exists now, so it should be easy enough to achieve the same "repaint small areas of the window, don't do global layout" workflow in a web-based app.
