Thanks for your bug report. This issue was probably first discussed around 20 years ago :-) This is a Windows limitation related to the WM_VSCROLL message for the scrollbar which can only handle a count of up to 65535. Although it's possible to get around it for a scrollbar control, there is, as far as I know, no way to get around it for a scrollbar that is part of a listbox. If anyone finds a way, let me know.
