If you take careful note of the wording used in bug 996038:

> 'moz-app-visually-complete': Emitted when the application designates that it is visually loaded, e.g. the "above-the-fold" content **exists in the DOM and has been marked as ready for display**: e.g. not `display: none`.

This event is used to mark when we have made this available in the DOM and ready for display, even if the display doesn't actually happen at that point. So your statement of:

> interpret the event as the element has existed in the dom tree but not guaranteed to be rendered

would be correct.