"The MouseEvent relatedTarget attribute must return the adjusted related target"
We need to compute related target ahead of time, since DOM operations in event listeners could affect the DOM structure.
*** Bug 20615 has been marked as a duplicate of this bug. ***
Made it clear at https://github.com/w3c/webcomponents/commit/fc19892ff6bf099323447f6a29808630ca66c348
Let me close.