app.css

.menu {
    position: fixed;
    left: 0;
    top: 0;
    width: 100%;
    height: 100%;
    overflow: hidden;
    pointer-events: none;
    z-index: 150;
}

.menu--visible {
    pointer-events: auto;
}

.app-menu {
    background-color: #fff;
    color: #fff;
    position: relative;
    max-width: 400px;
    width: 90%;
    height: 100%;
    box-shadow: 0 2px 6px rgba(0, 0, 0, 0.5);
    -webkit-transform: translateX(-103%);
            transform: translateX(-103%);
    display: flex;
    flex-direction: column;
    will-change: transform;
    z-index: 160;
    pointer-events: auto;            
}

.menu--visible .app-menu {
    -webkit-transform: none;
            transform: none;
}

.menu--animatable .app-menu {
    transition: all 130ms ease-in;
}

.menu--visible.menu--animatable  .app-menu {
    transition: all 330ms ease-out;
}

.menu:after {
    content: '';
    display: block;
    position: absolute;
    left: 0;
    top: 0;
    width: 100%;
    height: 100%;
    background: rgba(0,0,0,0.4);
    opacity: 0;
    will-change: opacity;
    pointer-events: none;
    transition: opacity 0.3s cubic-bezier(0,0,0.3,1);
}

.menu--visible.menu:after {
    opacity: 1;
    pointer-events: auto;
}

/* aux */

body {
  margin: 0;
}

.layout {
  width: 375px;
  height: 667px;
  background-color: #f5f5f5;
  position: relative;
}

.header {
  background-color: #ccc;
}

.menu-icon {
  content: "Menu";
  color: #fff;
  background-color: #666;
  width: 40px;
  height: 40px;
}

.app-menu {
  width: 300px;
  height: 667px;
  box-shadow: none;
  background-color: #ddd;
}

.menu:after {
  width: 375px;
  height: 667px;
}