html, body {
  margin: 0;
  padding: 0;
  width: 100%;     /* ❗ no uses 100vw */
  height: 100%;
  overflow-x: hidden;
  box-sizing: border-box;
}

.main-content {
  width: 100%;
  height: 100%;
  display: flex;
  flex-direction: column;
}

#main-content {
  transition: margin-left 0.3s ease, width 0.3s ease;
  height: 100vh;
  overflow: hidden;
  position: relative;
  box-sizing: border-box;
}
iframe#content-frame {
  width: 100%;
  height: 100%;
  border: none;
  display: block;
  transition: opacity 0.3s ease;
}