[PATCH 2/2] user32/listbox: Handle Mouse Wheel scrolling for multi-column listboxes properly

[Nikolay Sivov]

On 08/20/2018 09:53 PM, Gabriel Ivăncescu wrote:

> Multi-column listboxes scroll horizontally, so each wheel tick must go an entire page at a time instead of an item at a time. But we have to limit the amount of scrolling in this case to avoid "jumping over" columns, if the window is too small. This matches Windows behavior.
>
> The calculation has also been simplified to just integer arithmetic in all cases, since the division (the only operation with a fractional result) was immediately truncated to integer anyway, so the float cast was useless. This works fine because the multiplication is done before the division (parentheses have been added to emphasize this point).
>
> Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=22253
> Signed-off-by: Gabriel Ivăncescu <gabrielopcode at gmail.com>
> ---
>   dlls/user32/listbox.c | 19 +++++++++++++++----
>   1 file changed, 15 insertions(+), 4 deletions(-)
>
> diff --git a/dlls/user32/listbox.c b/dlls/user32/listbox.c
> index 991ab87..7256095 100644
> --- a/dlls/user32/listbox.c
> +++ b/dlls/user32/listbox.c
> @@ -2007,10 +2007,21 @@ static LRESULT LISTBOX_HandleMouseWheel(LB_DESCR *descr, SHORT delta )
>   
>       if (descr->wheel_remain && pulScrollLines)
>       {
> -        int cLineScroll;
> -        pulScrollLines = min((UINT) descr->page_size, pulScrollLines);
> -        cLineScroll = pulScrollLines * (float)descr->wheel_remain / WHEEL_DELTA;
> -        descr->wheel_remain -= WHEEL_DELTA * cLineScroll / (int)pulScrollLines;
> +        INT cLineScroll;
> +        if (descr->style & LBS_MULTICOLUMN)
> +        {
> +            pulScrollLines = min((UINT)descr->width / descr->column_width, pulScrollLines);
> +            pulScrollLines = max(1, pulScrollLines);
> +            cLineScroll = ((INT)pulScrollLines * descr->wheel_remain) / WHEEL_DELTA;
> +            descr->wheel_remain -= (cLineScroll * WHEEL_DELTA) / (INT)pulScrollLines;
> +            cLineScroll *= descr->page_size;
> +        }
> +        else
> +        {
> +            pulScrollLines = min((UINT) descr->page_size, pulScrollLines);
> +            cLineScroll = ((INT)pulScrollLines * descr->wheel_remain) / WHEEL_DELTA;
> +            descr->wheel_remain -= (cLineScroll * WHEEL_DELTA) / (INT)pulScrollLines;
> +        }
>           LISTBOX_SetTopItem( descr, descr->top_item - cLineScroll, TRUE );
>       }
>       return 0;

Float cast was a deliberate addition it seems, and relatively recent 
one, see f42cfc04eb05fa266cfae0c64452686a68c24151, CC'ing Ken.