When setting a Web Component to be draggable and starting the drag operation (via mouse down) on a child of that Web Component within its shadow DOM, the dragstart event is immediately followed by dragend and the drag operation is cancelled. Dragging the shadow DOM host itself works fine.
macOS 10.13, Safari 11.1
Starting the drag operation on either the host or the child works fine in Firefox and Chrome.
Drag-and-drop does not work when using Web Components
Thanks for the bug report! We've identified this bug as a duplicate of the bug 136836.
*** This bug has been marked as a duplicate of bug 136836 ***