AlanHK always forgets to mention that he's using the Windows Legacy version of Sigil that's still built with Qt5.6.2, Community WebKit, and Python 3.4.x. Not to say that the same thing won't happen in the official latest version of Sigil, but until it's verified that it does ... apples and oranges.
Besides ... "
Preview sometimes doesn't scroll" is hardly relevant to the portion of my post he quoted where I talked about how Preview and Book View both render the same code the same (because they use the same rendering engine).