If you scroll the Codeview or Preview window with the mouse wheel you can bring the other one into sync by clicking in the window you're scrolling - that's simple enough. If you're scrolling by clicking in a scroll bar (does anyone actually do that) or sliding a scroll button it's the same, click in the window you were scrolling to bring the other window into synch.
But if you use the up/down Arrow or Page up/down keys to scroll you have to grab the mouse and click in the window in which you were using up/down Arrow or Page up/down keys - not as simple, and its easy to click in the wrong one and 'lose' your place. I'm keyboard centric, so this happens to me quite often.
It maybe worth noting how the calibre editor behaves in this regard.
If its Codeview or Preview windows are scrolled with the mouse wheel the other one is not kept in sync - i.e it's the same as Sigil, as is the scroll bar behaviour.
However, the Preview window never seems to retain keyboard focus. even if one clicks in it, i.e. the Codeview window always retains keyboard focus. So, the up/down Arrow or Page up/down keys always operate on the Codeview window, and in this instance the Preview window is kept in synch with the Codeview window.
Not being able scroll the Preview window via the keyboard is a negative for me, and it's not compensated by the fact that the Preview window does sync with Codeview if the up/down keys are used. In fact I find it quite irritating; a page of code is often a lot more than a page of content. So, when I press Page down the Preview window may sit there like stunned mullet, or it may move by some apparently arbitrary amount.
BR