Dialogs (e.g. authentication prompts, payments, filepickers) require proper context for users to make decisions. Fullscreen, by definition is immersive, and removes the context that a user needs to make a decision. Therefore, whenever a page causes a dialog to be shown, that page will lose any HTML5 fullscreen that it has entered.
Docs: https://docs.google.com/document/d/1UhlQmjVk_AJuDLabB__mERqGQ3pKDtoZ-Qi9zudl424