Index: customform.inc
===================================================================
--- lcl/include/customform.inc	(revision 51106)
+++ lcl/include/customform.inc	(working copy)
@@ -1279,9 +1279,9 @@
   end;
 
 var
-  X, Y: integer;
+  X, Y, BorderW, BorderH: integer;
   p: TPosition;
-  AForm: TCustomForm;
+  AForm: TCustomForm; rcWindow, rcClient:TRect;
 begin
   if (Parent <> nil) or (ParentWindow <> 0) then exit;
 
@@ -1291,6 +1291,12 @@
   X := Left;
   Y := Top;
 
+  {$IFDEF Windows}
+    rcClient:=GetClientRect; GetWindowRect(Handle, rcWindow);
+    BorderW := (rcWindow.Right - rcWindow.Left) - rcClient.Right;
+    BorderH := (rcWindow.Bottom - rcWindow.Top) - rcClient.Bottom;
+  {$ENDIF}
+
   p := Position;
   if (Position = poMainFormCenter) and (Application.Mainform=nil) then
     p := poScreenCenter;
@@ -1297,13 +1303,13 @@
   case P of
     poDesktopCenter:
       begin
-        X := Screen.DesktopLeft + (Screen.DesktopWidth - Width) div 2;
-        Y := Screen.DesktopTop +(Screen.DesktopHeight - Height) div 2;
+        X := Screen.DesktopLeft + (Screen.DesktopWidth - (Width{$IFDEF Windows}+BorderW{$ENDIF})) div 2;
+        Y := Screen.DesktopTop +(Screen.DesktopHeight - (Height{$IFDEF Windows}+BorderH{$ENDIF})) div 2;
       end;
     poScreenCenter:
       begin
-        X := (Screen.Width - Width) div 2;
-        Y := (Screen.Height - Height) div 2;
+        X := (Screen.Width - (Width{$IFDEF Windows}+BorderW{$ENDIF})) div 2;
+        Y := (Screen.Height - (Height{$IFDEF Windows}+BorderH{$ENDIF})) div 2;
       end;
     poMainFormCenter,
     poOwnerFormCenter: