[Lazarus] Issue #15866: TIpHtmlPanel scrollbar not functional on Win64

[Juha Manninen]

http://bugs.freepascal.org/view.php?id=15866

Gerd Sager and Zeljan:
It is good to have some kind of fix for the issue, but IMO the problem
is elsewhere.
In my understanding the LM_xxx messages should map to WM_xxx messages
on Windows. The idea is to simulate the Windows message system in a
cross-platform way.
Thus, the problem is in wrong mapping of those messages somewhere.

Regards,
Juha