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

Gabriel Iv─âncescu gabrielopcode at gmail.com
Wed Aug 22 15:11:46 CDT 2018


On Wed, Aug 22, 2018 at 8:48 PM, Nikolay Sivov <nsivov at codeweavers.com> wrote:
> On 08/22/2018 06:49 PM, Ken Thomases wrote:
>
>> To be honest, I don't clearly remember why I put the float cast in there.
>> I think perhaps to avoid potential (if unlikely) integer overflow.  We want
>> something like MulDiv() but always rounding toward zero.  Perhaps just
>> casting to INT64 would be better.
>
>
> I like MulDiv better. Gabriel, do you think it's possible to get rid of some
> casts by changing variable types?
>

I don't think integer overflow is an issue. WHEEL_DELTA is defined as
120, which is not that big. That means the result would have to be
larger than 2^31/120 (in magnitude), i.e. 17895697  (this is the
result *after* the division). That's a pretty large number for the
amount of lines to scroll. I don't think it's ever possible in
practice :)

So honestly casting to INT64 sounds overkill to me.

As for reducing the amount of casts: I initially thought that
pulScrollLines can't be signed because the function
SystemParametersInfoW needs it unsigned (output parameter), however it
seems it takes a PVOID. So I can just set pulScrollLines itself to INT
and get rid of the casts. I'll prepare a v2 patch, also for the other
float casting patch series.



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