Created attachment 345636 [details] repro See attached repro case. I cannot find a way to make a custom element that successfully drags using draggable=true/ondragstart. The equivalent content not inside a custom element shadow DOM works fine.
dragStart does get called...
<rdar://problem/42542400>
Oh, this is an issue with shadow DOM, not custom elements.
*** This bug has been marked as a duplicate of bug 136836 ***