GitHub HTML Preview Shortcuts

Adds a professional button to view GitHub HTML files directly via htmlpreview.github.io
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// ==UserScript==
// @name         GitHub HTML Preview Shortcuts
// @namespace    html_preview.user.js
// @run-at       document-start
// @match        *://github.com/*
// @match        *://*.github.com/*
// @grant        none
// @version      1.0.1
// @author       Prashant Adesara
// @description  Adds a professional button to view GitHub HTML files directly via htmlpreview.github.io
// @license MIT
// ==/UserScript==

(function () {
  'use strict';

  const PREVIEW_BASE = 'https://htmlpreview.github.io/?';

  function isGitHubHtmlFile() {
      // Inspects location structure accurately during soft or hard navigation
      return location.hostname === 'github.com' &&
             location.pathname.includes('/blob/') &&
             location.pathname.endsWith('.html');
  }

  function ensureButton() {
      if (document.getElementById('__html_preview_btn_wrap__')) return;

      const wrap = document.createElement('div');
      wrap.id = '__html_preview_btn_wrap__';

      // Position it at the bottom right corner
      Object.assign(wrap.style, {
          position: 'fixed',
          right: '16px',
          bottom: '16px',
          zIndex: 2147483647,
      });
      // Position it at the top right corner
      /*
      Object.assign(wrap.style, {
          position: 'fixed',
          right: '20px',
          top: '20px',
          zIndex: 2147483647,
      });*/

      // Position it at the right center
      /*
      Object.assign(wrap.style, {
          position: 'fixed',
          right: '20px',
          top: '50%',
          transform: 'translateY(-50%)',
          zIndex: 2147483647,
      });*/

      const btn = document.createElement('button');
      btn.id = '__html_preview_btn__';
      btn.type = 'button';
      btn.innerHTML = 'Click to Preview';

      // Professional dark slate styling
      Object.assign(btn.style, {
          position: 'relative',
          padding: '8px 16px',
          borderRadius: '6px',
          border: '1px solid #334155',
          background: '#1e293b',
          color: '#f8fafc',
          fontSize: '14px',
          fontWeight: '600',
          fontFamily: '-apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif',
          cursor: 'pointer',
          boxShadow: '0 4px 6px -1px rgba(0, 0, 0, 0.15), 0 2px 4px -1px rgba(0, 0, 0, 0.1)',
          userSelect: 'none',
          transition: 'all 0.2s ease',
      });

      // Hover animations
      btn.addEventListener('mouseenter', () => {
          btn.style.background = '#4f46e5';
          btn.style.borderColor = '#6366f1';
      });
      btn.addEventListener('mouseleave', () => {
          btn.style.background = '#1e293b';
          btn.style.borderColor = '#334155';
      });

      // Click action to open preview URL in current tab
      btn.addEventListener('click', (e) => {
          e.preventDefault();
          location.href = PREVIEW_BASE + location.href;
      });

      wrap.appendChild(btn);

      // Append to documentElement safely at document-start execution
      (document.body || document.documentElement).appendChild(wrap);
  }

  function removeButton() {
      document.getElementById('__html_preview_btn_wrap__')?.remove();
  }

  function updateUI() {
      if (isGitHubHtmlFile()) {
          ensureButton();
      } else {
          removeButton();
      }
  }

  // Handles dynamic elements lifecycle changes on SPA page transitions
  function observe() {
      const init = () => {
          updateUI();

          // Observe the document body for mutations to catch SPA view re-renders instantly
          /*if (document.body) {
              new MutationObserver(() => {
                  updateUI();
              }).observe(document.body, { childList: true, subtree: true });
          }*/
          const title = document.querySelector('title');
          if (title) {
            new MutationObserver(updateUI).observe(title, { childList: true, subtree: true });
          }

          if (document.head) {
            new MutationObserver(updateUI).observe(document.head, { childList: true, subtree: true });
          }
      };

      // Fallback checks for safe execution environment attachment points
      if (document.readyState === 'loading') {
          document.addEventListener('DOMContentLoaded', init, { once: true });
      } else {
          init();
      }
  }

  observe();
})();