style.css

.drop-zone {
  width: 100px;
  height: 20px;
  border: 1px dashed #777;
  display: inline-block;
}

.receiving-drag {
  background-color: hotpink;
}

.upload {
  position: fixed;
  left: 0;
  right: 0;
  top: 0;
  height: 24px;
  border-bottom: 1px solid #ccc;
  padding: 5px;
}

.text-view {
  position: fixed;
  left: 0;
  width: 250px;
  top: 42px;
  bottom: 0;
  padding: 5px;
}

.text-view textarea {
  width: 100%;
  height: 100%;
  font-family: Courier, monospace;
  font-size: 16px;
}

.image-viewer {
  position: fixed;
  top: 42px;
  left: 260px;
  right: 0;
  bottom: 0;
  overflow-x: auto;
  overflow-y: auto;
}
.image-viewer.drag-hover {
  border: 4px dashed gray;
}

.box {
  border: 1px solid blue;
  color: red;
  font-size: 32px;
}
.box.selected {
  border: 2px solid red;
}

.help {
  margin: 20px;
  font-size: large;
  pointer-events: none;
}