In Chrome/Firefox, when a component with light DOM content is attached to the document, and a slot is created with a slotchange listener, the slot will raise an initial slotchange event. Safari does not do this.
The Chrome/Firefox behavior is useful because it allows a component to concentrate its initial content processing in a slotchange handler. To get consistent behavior, we have to normalize the slotchange behavior across browsers, which is cumbersome.
This is a duplicate of webkit.org/b/169718.
*** This bug has been marked as a duplicate of bug 169718 ***