Note: I've deleted this once, and edited it even after reposting it, apologies for rapid changes
I've been trying to piece together what was actually there based upon some historical sources [1], [2], [3] and it's quite murky to me. As far as I can tell, scroll-follows-mouse was not truly possible in Windows 95, because the custom MSH_MOUSEWHEEL event lacked a way to carry the keyboard state, and an unfocused window would not have received the keyboard events to infer it correctly, so it would have been broken in a lot of edge cases. Then, Windows 98 added the standard MS_MOUSEWHEEL event, which does carry the keyboard state, but only 32-bit applications are able to access it, so the O/S locked scrolling to the focused window. Finally, Windows 10 pulled the plug on 16-bit compatibility, and so could and did make true scroll-follows-mouse possible.
I've been trying to piece together what was actually there based upon some historical sources [1], [2], [3] and it's quite murky to me. As far as I can tell, scroll-follows-mouse was not truly possible in Windows 95, because the custom MSH_MOUSEWHEEL event lacked a way to carry the keyboard state, and an unfocused window would not have received the keyboard events to infer it correctly, so it would have been broken in a lot of edge cases. Then, Windows 98 added the standard MS_MOUSEWHEEL event, which does carry the keyboard state, but only 32-bit applications are able to access it, so the O/S locked scrolling to the focused window. Finally, Windows 10 pulled the plug on 16-bit compatibility, and so could and did make true scroll-follows-mouse possible.
[1]: https://devblogs.microsoft.com/oldnewthing/20080806-00/?p=21...
[2]: https://web.archive.org/web/20070503040017/http://msdn2.micr...
[3]: https://devblogs.microsoft.com/oldnewthing/20160420-00/?p=93...