.app {
  width: calc(100% - 16px);
  height: calc(100% - 16px);
  margin: 8px;
  background-color: darkgrey;
  overflow: hidden;
  position: relative;
}

.toolbox {
  position: absolute;
  max-width: calc(100% - 8px);
  top: 4px;
  left: 4px;
  max-height: calc(100% - 8px);
  z-index: 4;
  background-color: rgba(75, 83, 107, 0.5);
}

.toolbox > * {
  margin: 4px;
  display: block;
}

.toolbox button {
  background-color: rgba(75, 83, 107, 0.8);
  color: aliceblue;
}

.app {
  width: calc(100% - 16px);
  height: calc(100% - 16px);
  margin: 8px;
  background-color: darkgrey;
  overflow: hidden;
  position: relative;
}

.toolbox {
  position: absolute;
  max-width: calc(100% - 8px);
  top: 4px;
  left: 4px;
  max-height: calc(100% - 8px);
  z-index: 4;
  background-color: rgba(75, 83, 107, 0.5);
}

.toolbox > * {
  margin: 4px;
  display: block;
}

.toolbox button {
  background-color: rgba(75, 83, 107, 0.8);
  color: aliceblue;
}

.cell {
  cursor: crosshair;
}

Make the default grid cursor a crosshair.
Make the default grid cursor a crosshair.