[Kai771]

What will become gray depends on what was shown on the previous screen. You're only having in mind your case, where you only have a few lines on the second screen. There are situations where both screens are roughly the same size. Going back doesn't always mean that you'd want to read the whole page. What if you wanted to read only the top paragraph, that wasn't shown completely on the 2nd screen?

The function is called overlap shading. Whatever is "overlap" (content shown on previous screen) is shaded. Perfectly logical if you ask me. There is no such thing as "main part" - if it was shown on the previuos screen - it's overlap. It is gray to indicate that it was shown on previous screen. What you actually want is overlap shading working just in forward direction. It might suit you, but would probably not suit someone else.