[lilyball]

And what do "developers" expect from it? Because as a user I certainly remember that it used to be very common for website layout to jump around in an aggravating fashion when hiding the address bar.

These new units do not indicate that Apple's 100vh behavior is "troublesome", they indicate that mobile needs better viewport units than desktop does. Which is why there are three separate new viewport units, to correspond to the three different desired measurements. Even if Apple never changed 100vh behavior, these new set of units are a significant improvement.