ChatGPT Multi-line prompts by default - A user script for Tampermonkey et al (drafted by ChatGPT and fixed to actually work by me)

gpt-multiprompt.user.js

// ==UserScript==
// @name         ChatGPT Multiline Prompt Helper
// @namespace    http://tampermonkey.net/
// @version      1.2
// @description  Use Enter for newline, Ctrl+Enter to submit
// @match        https://chat.openai.com/*
// @match        https://chatgpt.com/c/*
// @grant        none
// ==/UserScript==

(function () {
    'use strict';

    const bindHandler = () => {
        const inputDiv = document.querySelector('#prompt-textarea[contenteditable="true"]');
        if (!inputDiv || inputDiv.dataset.bound === "true") {
            return;
        }

        inputDiv.dataset.bound = "true";

        inputDiv.addEventListener('keydown', function (e) {
            if (e.key === 'Enter' && !e.ctrlKey && !e.metaKey && !e.shiftKey) {
                e.preventDefault();
                e.stopPropagation();

                const shiftEnter = new KeyboardEvent('keydown', {
                    key: 'Enter',
                    code: 'Enter',
                    keyCode: 13,
                    which: 13,
                    bubbles: true,
                    cancelable: true,
                    shiftKey: true
                });

                inputDiv.dispatchEvent(shiftEnter);
            }

            // Ctrl+Enter submits as normal
        }, true); // capture = true ensures early interception
        observer.disconnect()
    };

    const observer = new MutationObserver(bindHandler);
    observer.observe(document.body, { childList: true, subtree: true });

    window.addEventListener('load', bindHandler);
})();