Fix nested drag sources

1. Only preventDefault() if *no* drag sources in the nesting chain handled drag start
2. When a nested drag sources begins drag, don't let its parents handle it too

modules/backends/HTML5.js

@@ -59,6 +59,13 @@ function preventDefaultNativeDropAction(e) {
   }
 }
 
+function handleTopDragStart(e) {
+  // If by this time no drag source reacted, tell browser not to drag.
+  if (!DragOperationStore.isDragging()) {
+    e.preventDefault();
+  }
+}
+
 function handleTopDragEnter(e) {
   // IE requires this to not show a nodrag icon over the container
   e.preventDefault();
@@ -118,6 +125,7 @@ var HTML5 = {
       return;
     }
 
+    window.addEventListener('dragstart', handleTopDragStart);
     window.addEventListener('dragenter', handleTopDragEnter);
     window.addEventListener('dragover', handleTopDragOver);
     window.addEventListener('dragleave', handleTopDragLeave);
@@ -129,6 +137,7 @@ var HTML5 = {
       return;
     }
 
+    window.removeEventListener('dragstart', handleTopDragStart);
     window.removeEventListener('dragenter', handleTopDragEnter);
     window.removeEventListener('dragover', handleTopDragOver);
     window.removeEventListener('dragleave', handleTopDragLeave);

modules/utils/createDragDropMixin.js

@@ -199,9 +199,7 @@ function createDragDropMixin(backend) {
 
     handleDragStart(type, e) {
       var { canDrag, beginDrag } = this._dragSources[type];
-
-      if (!canDrag(this)) {
-        e.preventDefault();
+      if (DragOperationStore.isDragging() || !canDrag(this)) {
         return;
       }
 
@@ -216,6 +214,10 @@ function createDragDropMixin(backend) {
         y: offsetFromClient.y - containerRect.top
       };
 
+      if (!dragPreview) {
+        dragPreview = containerNode;
+      }
+
       if (!effectsAllowed) {
         // Move is a sensible default drag effect.
         // Browser shows a drag preview anyway so we usually don't want "+" icon.