/* Opaque, independent scroll surfaces avoid repainting stacked glass layers. */
@media (min-width: 861px) {
  #app .sidebar, #app .main, #app .topbar, #app .msg, #app .composer {
    -webkit-backdrop-filter: none;
    backdrop-filter: none;
  }
  #app .sidebar { background: linear-gradient(180deg, #fff, #f1f6fc); }
  #app .main { background: #f6f9fd; }
  #app .topbar { background: #fff; }
  #app .lesson-view { background: #f1f5fb; }
  #app .pdf-wrap { background: #f1f5fb; }
  #app .pdf-image-viewer {
    background: #eaf0f7;
    contain: layout paint;
    isolation: isolate;
    box-shadow: none;
    border-radius: 12px;
    scroll-behavior: auto;
  }
  #app .pdf-page img {
    background: #fff;
    border-radius: 4px;
    box-shadow: 0 2px 6px #28456912;
  }
  #app .pdf-page-open, #app .pdf-page img {
    transform: none;
    transition: none;
    filter: none;
  }
  #app .messages { contain: layout paint; isolation: isolate; scroll-behavior: auto; }
  #app .msg { background: #fff; box-shadow: none; }
  #app .msg.user { background: #eaf0ff; }
  #app .composer { background: #fff; box-shadow: 0 2px 8px #2845690a; }
}