zsh-style typeahead: the rest of the last matching line, greyed out after the cursor, Ctrl-F to take it.

Drawn AFTER readline has drawn, on the tick following each keypress. readline draws on that same keypress and would paint over anything written first; the ghost is appended to its output and the cursor walked back over it, so the line readline believes it has is the line it has. Nothing here touches server.line, which is why an unaccepted suggestion cannot end up in what gets evaluated.

import { setupSuggestionBehaviors } from './completion.ts';

import type { REPLServer } from 'node:repl';

// Defined, not invoked: it listens on a live terminal.
function example(server: REPLServer) {
  setupSuggestionBehaviors(server); // ghost text on, Ctrl-F accepts
}