This question looks like How to keep cursor horizontal position while scrolling in VIM? but it's actually not.
Vim always tries to snap the cursor to the first non-whitespace character of the destination line while scrolling.
Consider this text:
A Line 2
Now the cursor points to 'A', when I press
<C-d> to scroll a half page, Vim will automatically snap the cursor to 'L' in the second line.
What I want is to remain the horizontal position of cursor unless I explicitly command Vim to do so (like 'h', 'l').