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LibWeb/HTML: Account for floating point error in step mismatch check

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This introduces a new `is_integral_multiple` method that replaces the
`fmod(...) != 0` check. This fixes some WPT-Tests, where the step
cannot be accurately represented as a double.
This commit is contained in:
Glenn Skrzypczak
2025-08-17 16:53:31 +02:00
committed by Jelle Raaijmakers
parent 0313b2d97e
commit d2cb8057c3
Notes: github-actions[bot] 2025-12-11 15:19:17 +00:00 
Author: https://github.com/skyz1
Commit: https://github.com/LadybirdBrowser/ladybird/commit/d2cb8057c37
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/5884
Reviewed-by: https://github.com/gmta ✅
Reviewed-by: https://github.com/konradekk
2 changed files with 24 additions and 6 deletions
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23
Libraries/LibWeb/HTML/HTMLInputElement.cpp
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@@ -61,6 +61,25 @@
#include <LibWeb/WebIDL/DOMException.h>
#include <LibWeb/WebIDL/ExceptionOr.h>
namespace {
bool is_integral_multiple(double value, double step)
{
VERIFY(step > 0);
// The step is so small, that the integral multiple closest to the value cannot be accurately expressed as a double
if (fabs(value) / ldexp(1.0, NumericLimits<double>::digits()) > step)
return true;
auto division_remainder = fabs(remainder(value, step));
// NOTE: There is no spec for the integral multiple. However, chromium uses tolerance so we comply here.
// https://github.com/chromium/chromium/blob/18aeefb6d0fe94e267c08e1aaeaf2632937f4ce2/third_party/blink/renderer/core/html/forms/step_range.cc#L67
double acceptable_error = round(step) == step ? 0 : step / static_cast<double>(1 << NumericLimits<float>::digits());
return division_remainder <= acceptable_error || division_remainder >= (step - acceptable_error);
}
}
namespace Web::HTML {
GC_DEFINE_ALLOCATOR(HTMLInputElement);
@@ -2949,7 +2968,7 @@ WebIDL::ExceptionOr<void> HTMLInputElement::step_up_or_down(bool is_down, WebIDL
// 7. If value subtracted from the step base is not an integral multiple of the allowed value step, then set value to the nearest value that,
// when subtracted from the step base, is an integral multiple of the allowed value step, and that is less than value if the method invoked was the stepDown() method, and more than value otherwise.
if (fmod(step_base() - value, allowed_value_step) != 0) {
if (!is_integral_multiple(step_base() - value, allowed_value_step)) {
if (is_down) {
value = step_base() + floor((value - step_base()) / allowed_value_step) * allowed_value_step;
} else {
@@ -3645,7 +3664,7 @@ bool HTMLInputElement::is_number_mismatching_step(double number) const
double allowed_value_step = *maybe_allowed_value_step;
// and that number subtracted from the step base is not an integral multiple of the allowed value step, the element
// is suffering from a step mismatch.
return fmod(step_base() - number, allowed_value_step) != 0;
return !is_integral_multiple(step_base() - number, allowed_value_step);
}
// https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#suffering-from-bad-input