Bug 1636740 - Block CMemStream::Read only if the drop target initiated an async operation. r=aklotz

If the drop target did not initiate an async operation, `DoDragDrop` will
block on the main thread until the data transfer completes. Although the
`ObtainCachedIconFile` callback itself will run off the main thread,
`ObtainCachedIconFile` will end up with calling `NotifyIconObservers` that
heavily relies on the main thread. So `ObtainCachedIconFile` will have no
chance to run the callback.

To complete the data transfer, however, the `ObtainCachedIconFile` callback
must signal the event object to unblock `CMemStream::Read`. Deadlock.

Differential Revision: https://phabricator.services.mozilla.com/D75531

widget/windows/nsDataObj.cpp

@@ -1360,6 +1360,11 @@ nsDataObj ::GetFileContentsInternetShortcut(FORMATETC& aFE, STGMEDIUM& aSTG) {
   ::GlobalUnlock(globalMem.get());
 
   if (aFE.tymed & TYMED_ISTREAM) {
+    if (!mIsInOperation) {
+      // The drop target didn't initiate an async operation.
+      // We can't block CMemStream::Read.
+      event = nullptr;
+    }
     RefPtr<IStream> stream =
         new CMemStream(globalMem.disown(), totalLen, event.forget());
     stream.forget(&aSTG.pstm);