I am not sure that changing an established rule (one that could affect many users negatively) is a good idea. Anyone who is using the current functionality as it works today would have their implementations broken if the logic were to change in the way you are suggesting.
If you are trying to track that your menu is being opened, I would either use a Click Trigger on a CSS Selector (depending on the complexity), or get my developers to trigger a custom JS event (one for open, one for close - if you need that level of granularity) and create a custom code trigger listening for those events.
Core - Enters Viewport isn't intended to be used in the way you describe, to detect elements becoming visible... it's intended to detect that an element is within the viewport; such as detecting when the user scrolls down the page and sees a specific element - like content that follows an article to understand if the user is scrolling through the entire "main content" - this logic may very well need to work with "hidden" elements based on how users have implemented it (like hidden pixels or divs at multiple places down the page).
