userscripts/emdash-highlighter.user.js

// ==UserScript==
// @name         Em Dash Highlighter
// @namespace    Reichard
// @version      1.0.0
// @description  Detects em dashes on the page, highlights them, and shows a popup on hover/tap
// @author       Evan Reichard
// @match        *://*/*
// @grant        GM_addStyle
// @run-at       document-idle
// ==/UserScript==

(function () {
  'use strict';

  // ─── Constants ────────────────────────────────────────────────────────────

  const EM_DASH = '\u2014';
  const EMDASH_REGEX = /\u2014/g;
  const POPUP_ID = 'emdash-popup';

  // Tags whose text content we should never touch
  const SKIP_TAGS = new Set([
    'SCRIPT', 'STYLE', 'NOSCRIPT', 'TEXTAREA', 'INPUT',
    'SELECT', 'BUTTON', 'PRE', 'SVG', 'MATH',
  ]);

  // ─── Styles ───────────────────────────────────────────────────────────────

  GM_addStyle(`
    .emdash-highlight {
      display: inline;
      padding: 0 1px;
      border-radius: 2px;
      cursor: help;
      animation: emdash-hue 4s linear infinite;
      background-color: hsl(0, 80%, 70%);
      position: relative;
    }

    @keyframes emdash-hue {
      0%   { background-color: hsl(0,   80%, 70%); }
      25%  { background-color: hsl(90,  80%, 70%); }
      50%  { background-color: hsl(180, 80%, 70%); }
      75%  { background-color: hsl(270, 80%, 70%); }
      100% { background-color: hsl(360, 80%, 70%); }
    }

    #${POPUP_ID} {
      position: fixed;
      z-index: 2147483647;
      background: #1e1e2e;
      color: #cdd6f4;
      border: 1px solid #89b4fa;
      border-radius: 6px;
      padding: 6px 10px;
      font: 13px/1.4 monospace;
      pointer-events: none;
      white-space: nowrap;
      box-shadow: 0 4px 12px rgba(0,0,0,0.5);
      opacity: 0;
      transition: opacity 0.15s ease;
      max-width: 280px;
      white-space: normal;
    }

    #${POPUP_ID}.visible {
      opacity: 1;
    }
  `);

  // ─── Popup singleton ──────────────────────────────────────────────────────

  const popup = document.createElement('div');
  popup.id = POPUP_ID;
  popup.innerHTML = `
    <strong style="color:#89b4fa">Em Dash</strong> &mdash; U+2014
  `;
  document.documentElement.appendChild(popup);

  let hideTimer = null;

  function showPopup(anchorEl) {
    clearTimeout(hideTimer);
    const rect = anchorEl.getBoundingClientRect();
    const popupHeight = popup.offsetHeight || 80;
    const popupWidth  = popup.offsetWidth  || 220;

    let top  = rect.top - popupHeight - 8;
    let left = rect.left + rect.width / 2 - popupWidth / 2;

    // Flip below if not enough room above
    if (top < 4) top = rect.bottom + 8;

    // Clamp horizontally
    left = Math.max(6, Math.min(left, window.innerWidth - popupWidth - 6));

    popup.style.top  = `${top}px`;
    popup.style.left = `${left}px`;
    popup.classList.add('visible');
  }

  function hidePopup(delay = 120) {
    hideTimer = setTimeout(() => popup.classList.remove('visible'), delay);
  }

  // ─── Span factory ─────────────────────────────────────────────────────────

  function createSpan() {
    const span = document.createElement('span');
    span.className = 'emdash-highlight';
    span.setAttribute('aria-label', 'Em dash character');
    span.textContent = EM_DASH;

    // Desktop hover
    span.addEventListener('mouseenter', () => showPopup(span));
    span.addEventListener('mouseleave', () => hidePopup());

    // Mobile tap — show/hide without bubbling
    span.addEventListener('touchend', (e) => {
      e.stopPropagation();
      e.preventDefault();
      if (popup.classList.contains('visible')) {
        hidePopup(0);
      } else {
        showPopup(span);
        // Auto-hide after 3 s on mobile
        hideTimer = setTimeout(() => popup.classList.remove('visible'), 3000);
      }
    }, { passive: false });

    // Prevent click bubbling (e.g. accordion toggles, links)
    span.addEventListener('click', (e) => {
      e.stopPropagation();
    });

    return span;
  }

  // ─── Core processor ───────────────────────────────────────────────────────

  /**
   * Walk a DOM subtree and wrap every em dash in a text node with a span.
   * We operate on Text nodes directly to avoid serialising/deserialising HTML.
   */
  function processNode(root) {
    // TreeWalker is faster than querySelectorAll for text nodes
    const walker = document.createTreeWalker(
      root,
      NodeFilter.SHOW_TEXT,
      {
        acceptNode(node) {
          const parent = node.parentElement;
          if (!parent) return NodeFilter.FILTER_REJECT;
          if (SKIP_TAGS.has(parent.tagName)) return NodeFilter.FILTER_REJECT;
          if (parent.isContentEditable) return NodeFilter.FILTER_REJECT;
          if (parent.classList.contains('emdash-highlight')) return NodeFilter.FILTER_REJECT;
          if (!node.nodeValue.includes(EM_DASH)) return NodeFilter.FILTER_SKIP;
          return NodeFilter.FILTER_ACCEPT;
        },
      }
    );

    // Collect first — mutating the DOM while walking breaks the walker
    const textNodes = [];
    let node;
    while ((node = walker.nextNode())) textNodes.push(node);

    for (const textNode of textNodes) {
      splitAndWrap(textNode);
    }
  }

  /**
   * Split a text node around em dashes and replace it with a fragment
   * containing plain text nodes and highlight spans.
   */
  function splitAndWrap(textNode) {
    const text = textNode.nodeValue;
    const parts = text.split(EMDASH_REGEX);

    if (parts.length <= 1) return; // no em dash found

    const fragment = document.createDocumentFragment();

    parts.forEach((part, i) => {
      if (part) fragment.appendChild(document.createTextNode(part));
      if (i < parts.length - 1) fragment.appendChild(createSpan());
    });

    textNode.parentNode.replaceChild(fragment, textNode);
  }

  // ─── Mutation observer ────────────────────────────────────────────────────

  const observer = new MutationObserver((mutations) => {
    for (const mutation of mutations) {
      if (mutation.type === 'characterData') {
        // Text node changed in place
        const node = mutation.target;
        if (
          node.nodeValue?.includes(EM_DASH) &&
          node.parentElement &&
          !SKIP_TAGS.has(node.parentElement.tagName) &&
          !node.parentElement.classList.contains('emdash-highlight')
        ) {
          splitAndWrap(node);
        }
      } else if (mutation.type === 'childList') {
        for (const added of mutation.addedNodes) {
          if (added.nodeType === Node.TEXT_NODE) {
            if (
              added.nodeValue?.includes(EM_DASH) &&
              added.parentElement &&
              !SKIP_TAGS.has(added.parentElement.tagName) &&
              !added.parentElement.classList.contains('emdash-highlight')
            ) {
              splitAndWrap(added);
            }
          } else if (added.nodeType === Node.ELEMENT_NODE) {
            processNode(added);
          }
        }
      }
    }
  });

  // ─── Global tap-outside to dismiss popup ──────────────────────────────────

  document.addEventListener('touchstart', () => hidePopup(0), { passive: true });

  // ─── Init ─────────────────────────────────────────────────────────────────

  function init() {
    processNode(document.body);

    observer.observe(document.body, {
      childList: true,
      subtree: true,
      characterData: true,
    });
  }

  // document-idle fires after DOMContentLoaded, but guard anyway
  if (document.readyState === 'loading') {
    document.addEventListener('DOMContentLoaded', init);
  } else {
    init();
  }
})();