[PATCH] dix: make input clipping respect subpixels (#24187)

[Simon Thum]

Peter Hutterer wrote:
> @@ -587,16 +587,41 @@ clipAxis(DeviceIntPtr pDev, int axisNum, int *val)
>  
>      /* If a value range is defined, clip. If not, do nothing */
A nit: This is no longer true then.
>      if (axis->max_value <= axis->min_value)
> -        return;
> +    {
> +        /* Special case: for relative devices with no valid axis range, we
> +         * still need to clip off the remainder to avoid negative root
> +         * coordinates. We do NOT set val to 0, otherwise we can't cross
> +         * screens anymore. */
> +        if (*val <= axis->min_value && remainder && (*remainder < 0.0f))
> +            *remainder = 0.0f;
Errrm... that's a bug. You're killing the remainder based on an
undefined axis range. That can't really be it?

Supposed the axis is screen (which we first checked it can't be, since
screen axis would have a valid range) it would be sensible to me to add
the analog line:

if (*val >= axis->max_value && remainder && (*remainder > 0.0f))
    *remainder = 0.0f;

so we actually hold what's intended:
"The clipping rules intend to avoid enlarging the
valuator range by the remainder range, so assumptions
one may have made stay valid."

Second, it should really be:

if (*val >= axis->max_value && remainder && (*remainder > 0.0f || *val >
axis->min_value))

and vice versa, because if we're bigger/smaller (i.e. not just equal)
the remainder always must be zeroed. That's basically why the patch is
so complicated compared to what it does.

But I think the real solution must involve screen limits, which seems to
happen somewhere else.

Cheers,

Simon