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https://fullscreen.spec.whatwg.org/#concept-fullscreen-add [[ To add an element on a document's fullscreen element stack, add, or move if already present, it on top of document's browsing context's top layer, and then add, or move if already present, it on top of document's fullscreen element stack. ]] Since the spec includes the following items in the "fullscreen element ready check" algorithm: [[ element's node document's fullscreen element stack is either empty or its top element is an inclusive ancestor of element. ]] I don't think it is possible that an element already in the stack could be added to the stack again. Hence I suggest to remove the "or move if already present".
For top layer this is definitely untrue as that concept is shared with other features, such as <dialog>. Why do you think it is true for the fullscreen element stack. It could be both non-empty and filled with elements that are not inclusive ancestors of the element in question, no?
(In reply to Anne from comment #1) > For top layer this is definitely untrue as that concept is shared with other > features, such as <dialog>. OK > Why do you think it is true for the fullscreen element stack. It could be > both non-empty and filled with elements that are not inclusive ancestors of > the element in question, no? Because in this case, the requestFullscreen() method will fail in "fullscreen element ready check" step, which stops this method from adding element to the stack.
Yeah sorry, I guess you're right. I'm going to hold off on fixing this until I get a chance to fix the other outstanding issues.
This was fixed by merging the two concepts in bug 27865.