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Description
When you call observe on an element, IntersectionObserver assumes it's hidden. So, if it's actually hidden, the callback never fires. In some cases, you want the callback to fire if it's hidden and not if it's visible. I expect there are also cases where you always want the callback to fire after you call observer.
I propose that we add a new value to the InstersectionObserver constructor's options argument to control this.
new IntersectionObserver(callback, { initialState: hidden|visible|none })
hidden == assume hidden and don't fire the callback if it is hidden in the frame after observe is called.
visible == the opposite of hidden
none == always fire the callback in the frame after observe is called