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Remove max-width constraints from LMArena chat containers for a full-width reading experience. Works across all chat modes.
// ==UserScript==
// @name LMArena Chat Full Width
// @name:zh-TW LMArena 全寬度聊天訊息
// @namespace http://tampermonkey.net/
// @version 1.0.0
// @description Remove max-width constraints from LMArena chat containers for a full-width reading experience. Works across all chat modes.
// @description:zh-TW 讓 LMArena 的聊天訊息延展至全螢幕寬度，獲得更舒適的閱讀體驗。
// @author Community
// @match https://lmarena.ai/*
// @match https://beta.lmarena.ai/*
// @icon https://lmarena.ai/favicon.ico
// @grant GM_addStyle
// @run-at document-start
// @license MIT
// ==/UserScript==
/**
* ╔═══════════════════════════════════════════════════════════════════════════╗
* ║ LMArena Chat Full Width ║
* ╠═══════════════════════════════════════════════════════════════════════════╣
* ║ DESIGN PHILOSOPHY: ║
* ║ • CSS-first approach for performance and instant application ║
* ║ • JavaScript fallback for edge cases (inline styles, dynamic content) ║
* ║ • Minimal runtime overhead with smart debouncing ║
* ║ • Comprehensive selector coverage based on real DOM analysis ║
* ╚═══════════════════════════════════════════════════════════════════════════╝
*/
(function () {
'use strict';
/* ═══════════════════════════════════════════════════════════════════════════
* CONFIGURATION
* ═══════════════════════════════════════════════════════════════════════════ */
const CONFIG = Object.freeze({
// Maximum ancestor levels to traverse when clearing max-width
MAX_ANCESTOR_DEPTH: 12,
// Debounce interval for mutation handling (ms) — roughly 1 frame at 60fps
DEBOUNCE_MS: 16,
// Selectors for Sentry-instrumented message components
MESSAGE_COMPONENTS: [
'[data-sentry-component="UserMessage"]',
'[data-sentry-component="BotMessage"]',
'[data-sentry-component="AIMessage"]',
'[data-sentry-component="ParallelMessageGroup"]',
],
// Additional structural selectors observed in LMArena DOM
STRUCTURAL_SELECTORS: [
'#chat-area',
'main',
'main ol',
'main ol > li',
'main ol > div',
],
});
/* ═══════════════════════════════════════════════════════════════════════════
* CSS STYLES
* Injected at document-start to prevent FOUC (Flash of Unstyled Content)
* ═══════════════════════════════════════════════════════════════════════════ */
const CSS = /* css */ `
/*******************************************************************************
* LMArena Chat Full Width — Injected Styles
* Priority: !important ensures override of Tailwind/inline styles
******************************************************************************/
/* ═══════════════════════════════════════════════════════════════════════════
Layer 1: Primary Layout Containers
═══════════════════════════════════════════════════════════════════════════ */
#chat-area,
main,
main > div {
max-width: none !important;
}
/* ═══════════════════════════════════════════════════════════════════════════
Layer 2: Sentry-Instrumented Message Components
These are the core chat message wrappers identified via data attributes
═══════════════════════════════════════════════════════════════════════════ */
[data-sentry-component="UserMessage"],
[data-sentry-component="BotMessage"],
[data-sentry-component="AIMessage"],
[data-sentry-component="ParallelMessageGroup"] {
max-width: none !important;
}
/* ═══════════════════════════════════════════════════════════════════════════
Layer 3: Parent Container Targeting via :has()
Removes constraints from wrapper divs that contain message components
═══════════════════════════════════════════════════════════════════════════ */
/* Direct parent */
div:has(> [data-sentry-component="UserMessage"]),
div:has(> [data-sentry-component="BotMessage"]),
div:has(> [data-sentry-component="AIMessage"]),
div:has(> [data-sentry-component="ParallelMessageGroup"]) {
max-width: none !important;
}
/* Grandparent (handles nested wrapper patterns) */
div:has(> div > [data-sentry-component="UserMessage"]),
div:has(> div > [data-sentry-component="BotMessage"]),
div:has(> div > [data-sentry-component="AIMessage"]),
div:has(> div > [data-sentry-component="ParallelMessageGroup"]) {
max-width: none !important;
}
/* Great-grandparent (deep nesting edge cases) */
div:has(> div > div > [data-sentry-component]) {
max-width: none !important;
}
/* ═══════════════════════════════════════════════════════════════════════════
Layer 4: Tailwind CSS Utility Class Override
Catches max-w-sm, max-w-md, max-w-lg, max-w-xl, max-w-2xl, etc.
═══════════════════════════════════════════════════════════════════════════ */
main [class*="max-w-"],
#chat-area [class*="max-w-"],
div[class*="max-w-"]:has([data-sentry-component]) {
max-width: none !important;
}
/* ═══════════════════════════════════════════════════════════════════════════
Layer 5: Chat List Structures
Common patterns observed in LMArena's chat layout
═══════════════════════════════════════════════════════════════════════════ */
/* Ordered list used for message history */
main ol[class*="flex-col"],
main ol[class*="flex-col-reverse"] {
max-width: none !important;
}
/* List items and dividers */
main ol > li,
main ol > div {
max-width: none !important;
}
/* ═══════════════════════════════════════════════════════════════════════════
Layer 6: Flex Container Overrides
Ensures flex children can expand fully
═══════════════════════════════════════════════════════════════════════════ */
main [class*="flex"][class*="w-full"] {
max-width: none !important;
}
/* Message content areas */
[data-sentry-component] [class*="prose"],
[data-sentry-component] [class*="markdown"] {
max-width: none !important;
}
`;
/* ═══════════════════════════════════════════════════════════════════════════
* UTILITY FUNCTIONS
* ═══════════════════════════════════════════════════════════════════════════ */
/**
* Injects CSS into the document using the most reliable method available.
* @param {string} cssText - The CSS rules to inject
*/
function injectStyles(cssText) {
// Prefer GM_addStyle for proper userscript style management
if (typeof GM_addStyle === 'function') {
GM_addStyle(cssText);
return;
}
// Fallback: Create and inject a <style> element
const style = document.createElement('style');
style.setAttribute('data-injected-by', 'lmarena-fullwidth');
style.textContent = cssText;
// Insert at the earliest possible point
const target = document.head || document.documentElement;
if (target.firstChild) {
target.insertBefore(style, target.firstChild);
} else {
target.appendChild(style);
}
}
/**
* Creates a debounced version of a function.
* Uses requestAnimationFrame for optimal performance.
* @param {Function} fn - The function to debounce
* @returns {Function} - The debounced function
*/
function rafDebounce(fn) {
let frameId = null;
return function debounced(...args) {
if (frameId !== null) return;
frameId = requestAnimationFrame(() => {
fn.apply(this, args);
frameId = null;
});
};
}
/* ═══════════════════════════════════════════════════════════════════════════
* RUNTIME MAX-WIDTH ENFORCEMENT
* JavaScript fallback for cases CSS cannot handle (e.g., inline styles,
* dynamically computed styles, or deeply nested structures)
* ═══════════════════════════════════════════════════════════════════════════ */
/**
* Traverses the DOM upward from message components, forcibly removing
* any max-width constraints encountered.
*/
function enforceMaxWidthRemoval() {
const selector = CONFIG.MESSAGE_COMPONENTS.join(',');
const messageNodes = document.querySelectorAll(selector);
if (messageNodes.length === 0) return;
messageNodes.forEach((node) => {
traverseAndClearMaxWidth(node);
});
}
/**
* Walks up the DOM tree from a starting node, clearing max-width on each ancestor.
* @param {Element} startNode - The node to start from
*/
function traverseAndClearMaxWidth(startNode) {
let current = startNode;
let depth = 0;
while (current && current !== document.body && depth < CONFIG.MAX_ANCESTOR_DEPTH) {
clearMaxWidthIfConstrained(current);
current = current.parentElement;
depth++;
}
}
/**
* Clears max-width on an element if it has a constraining value.
* @param {Element} element - The element to check and potentially modify
*/
function clearMaxWidthIfConstrained(element) {
// Skip if already processed
if (element.dataset.fullwidthProcessed === 'true') return;
const computedStyle = getComputedStyle(element);
const maxWidth = computedStyle.maxWidth;
// Check if there's an actual constraint to remove
const isConstrained = maxWidth &&
maxWidth !== 'none' &&
maxWidth !== '0px' &&
!maxWidth.startsWith('0');
if (isConstrained) {
element.style.setProperty('max-width', 'none', 'important');
element.dataset.fullwidthProcessed = 'true';
}
}
/* ═══════════════════════════════════════════════════════════════════════════
* DOM MUTATION OBSERVER
* Monitors for dynamically added/modified chat messages
* ═══════════════════════════════════════════════════════════════════════════ */
/**
* Creates and configures a MutationObserver to watch for DOM changes.
* @returns {MutationObserver} - The configured observer
*/
function createMutationObserver() {
const debouncedEnforce = rafDebounce(enforceMaxWidthRemoval);
return new MutationObserver((mutations) => {
// Quick check: only process if mutations might be relevant
const hasRelevantMutation = mutations.some((mutation) =>
mutation.type === 'childList' && mutation.addedNodes.length > 0
);
if (hasRelevantMutation) {
debouncedEnforce();
}
});
}
/**
* Starts observing the most appropriate container for DOM changes.
* @param {MutationObserver} observer - The observer to start
*/
function startObserving(observer) {
// Prefer the most specific container to reduce noise
const observationTarget =
document.querySelector('#chat-area') ||
document.querySelector('main') ||
document.body;
observer.observe(observationTarget, {
childList: true,
subtree: true,
// attributes: false — we don't need to watch attribute changes
// characterData: false — we don't need to watch text content changes
});
}
/* ═══════════════════════════════════════════════════════════════════════════
* INITIALIZATION
* ═══════════════════════════════════════════════════════════════════════════ */
/**
* Main initialization function.
* Orchestrates style injection and observer setup.
*/
function initialize() {
// ─────────────────────────────────────────────────────────────────────────
// Phase 1: Immediate CSS Injection
// Runs at document-start, before any content renders
// ─────────────────────────────────────────────────────────────────────────
injectStyles(CSS);
// ─────────────────────────────────────────────────────────────────────────
// Phase 2: DOM-Ready Actions
// Set up observer and run initial enforcement once DOM is available
// ─────────────────────────────────────────────────────────────────────────
const observer = createMutationObserver();
const onDomReady = () => {
enforceMaxWidthRemoval();
startObserving(observer);
};
if (document.readyState === 'loading') {
document.addEventListener('DOMContentLoaded', onDomReady, { once: true });
} else {
// DOM already available (e.g., script loaded late)
onDomReady();
}
// ─────────────────────────────────────────────────────────────────────────
// Phase 3: Post-Load Sweep
// Catches any late-loading content or async-rendered components
// ─────────────────────────────────────────────────────────────────────────
window.addEventListener('load', enforceMaxWidthRemoval, { once: true });
}
/* ═══════════════════════════════════════════════════════════════════════════
* SCRIPT ENTRY POINT
* ═══════════════════════════════════════════════════════════════════════════ */
initialize();
})();