More to the point, the indicator doesn’t need to always be visible to be useful. Appearing only when the viewport is noving works fine on the iPhone, for instance, because slighly moving the viewpoint to reveal it is a convenient action, and less well on desktop PCs where your hands are less likely to remain near movement controls at all times.

It’s also applicaction specific: I care a lot more about relative viewport size and position when reading a long document than I do when I’m entering commands at the bottom of a terminal’s scrollback buffer.