Replace deprecated event.srcElement with event.target

partials/GridEditor.vue

@@ -214,7 +214,7 @@
           event.dataTransfer.setData('text/plain', JSON.stringify(cell))
           event.dataTransfer.setDragImage(nullImage, 0, 0)
           let elContainerBoundingBox = this.$el.getBoundingClientRect()
-          let elBoundingBox = event.srcElement.getBoundingClientRect()
+          let elBoundingBox = event.target.getBoundingClientRect()
           let offset = {
             x: (event.clientX - elContainerBoundingBox.x) - (elBoundingBox.x - elContainerBoundingBox.x),
             y: (event.clientY - elContainerBoundingBox.y) - (elBoundingBox.y - elContainerBoundingBox.y)