Delphi 7 on Windows 7
I want to scroll down a list (TElTree) using the mouse wheel... so I start writing some code in the ElTree's OnMouseWheelDown event. Then I notice that it's scrolling 2 rows at once. The mouse wheel settings in Control Panel are set to scroll only 1 line at a time. If I place a breakpoint inside the event handler, I discover that the event handler is itself being executed twice in quick succession.... Why? How to ensure it only executes once (code please)?
When the wheel scrolls, the OS sends WM_MOUSEWHEEL messages. The high-order word of the wParam parameter indicates how far the wheel has turned. If it has turned one "click," then its value will be 120, or WHEEL_DELTA. But it might be less than that if your scroll wheel recognizes scrolling less than a click's worth.
Correct WM_MOUSEWHEEL message handlers need to consider that parameter and either keep a "scrolling accumulator" to keep track of how far the wheel has scrolled or else have the ability to scroll less than a full line. Message handlers that assume that each message signifies a full click of the wheel will be sorry.