I simply added a custom-specific iView in the Work Area in the Layout of the Desktop Innerpage.
Now, I have a vertical scrollbar on almost all pages of the Portal.
My analysis is that the Work Area complelety ignores the custom-specific iView (the Desktop Innerpage height type is set to Automatic) when it calculates the available height. I tried different settings (Fixed, Full-page) without success.
Hope there is a solution that do not imply modifying or copying a standard Portal component (be it the Desktop Innerpage, the Navigation Layout (with Navigation Area / Work Area)... 😕