body {
  margin: 0;
}
.titlebar {
  background: blue;
  color: white;
  display: flex;
  justify-content: center;
  align-items: center;
  app-region: drag;
  
  margin-left: env(titlebar-area-x, 0);
  width:       env(titlebar-area-width, 100%);
  height:      env(titlebar-area-height, 30px);
  box-sizing: border-box;
  border: 1px dashed red;
}