From 178c43a3f6aed32ba5cf4a873f3b9af5b83701d5 Mon Sep 17 00:00:00 2001
From: bart <9132501-flyingsheep@users.noreply.gitlab.com>
Date: Mon, 25 Apr 2016 10:08:00 +0000
Subject: [PATCH] Win32: fix scrolling issue in ListBox with huge amount of
 items. Patch by Michl. Issue #0023914.

git-svn-id: trunk@52246 -
---
 lcl/interfaces/win32/win32callback.inc | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/lcl/interfaces/win32/win32callback.inc b/lcl/interfaces/win32/win32callback.inc
index e51896774d..d532d713d0 100644
--- a/lcl/interfaces/win32/win32callback.inc
+++ b/lcl/interfaces/win32/win32callback.inc
@@ -815,6 +815,14 @@ begin
   if LMScroll.Pos < High(LMScroll.SmallPos)
   then LMScroll.SmallPos := LMScroll.Pos
   else LMScroll.SmallPos := High(LMScroll.SmallPos);
+
+  if Assigned(lWinControl) and (lWinControl is TCustomListbox) and (LMsg = LM_VSCROLL) then
+  begin
+    // WM_VSCROLL message carries only 16 bits of scroll box position data.
+    // This workaround is needed, to scroll higher than a position value of 65536.
+    WinProcess := False;
+    TCustomListBox(lWinControl).TopIndex := LMScroll.Pos;
+  end;
 end;
 
 // FlashWindowEx is not (yet) in FPC