Fix crash on drag-and-drop over panel

The fake drag-and-drop events caused by the panel reuse the real event's
mimeData, and were handled asynchronously. By the time the fake event
has been dispatched from the event loop, the mimeData may already have
been freed. Send the fake events synchronously again.

A guard is added to avoid the original potential bug of infinite recursion.

BUG: 398440


(cherry picked from commit c19d8de7)