Refactor step handling in <input>

stepMismatch currently has its own way of checking whether the value is
aligned, even though the _isStepAligned function was written for this
very purpose. Move the decimal.js-based modulo check to _isStepAligned,
and make stepMismatch use that function instead.

Also rename the parameter for _stepAlign so that it makes more sense.

lib/jsdom/living/nodes/HTMLInputElement-impl.js

@@ -144,11 +144,12 @@ class HTMLInputElementImpl extends HTMLElementImpl {
   }
 
   _isStepAligned(v) {
-    const relativeDistance = v - this._stepBase;
-    return (relativeDistance % this._allowedValueStep) === 0;
+    return new Decimal(v).minus(this._stepBase)
+      .modulo(this._allowedValueStep)
+      .isZero();
   }
 
-  _stepAlign(v, isPositive) {
+  _stepAlign(v, roundUp) {
     if (this._isStepAligned(v)) {
       return v;
     }
@@ -157,7 +158,7 @@ class HTMLInputElementImpl extends HTMLElementImpl {
     const stepIntervalCount = Math.trunc(relativeDistance / this._allowedValueStep);
     let candidate = stepIntervalCount * this._allowedValueStep;
 
-    if (isPositive) {
+    if (roundUp) {
       if (v > candidate) {
         candidate += this._allowedValueStep;
       }
@@ -544,29 +545,24 @@ class HTMLInputElementImpl extends HTMLElementImpl {
     const valueBeforeStepping = value;
 
     if (!this._isStepAligned(value)) {
-      value = this._stepAlign(value, isUp);
+      value = this._stepAlign(value, /* roundUp = */ isUp);
     }
 
     let delta = n * allowedValueStep;
     if (!isUp) {
       delta *= -1;
     }
-
     value += delta;
 
     if (min !== null && value < min) {
-      value = this._stepAlign(min, true);
+      value = this._stepAlign(min, /* roundUp = */ true);
     }
 
     if (max !== null && value > max) {
-      value = this._stepAlign(max, false);
+      value = this._stepAlign(max, /* roundUp = */ false);
     }
 
-    if (isUp) {
-      if (value < valueBeforeStepping) {
-        return;
-      }
-    } else if (value > valueBeforeStepping) {
+    if (isUp ? value < valueBeforeStepping : value > valueBeforeStepping) {
       return;
     }
 
@@ -1102,11 +1098,7 @@ class HTMLInputElementImpl extends HTMLElementImpl {
           }
 
           const number = this._parsedValue;
-          const isIntegralMultiple =
-            new Decimal(number - this._stepBase)
-              .modulo(allowedValueStep)
-              .isZero();
-          return !isIntegralMultiple;
+          return !this._isStepAligned(number);
         },
 
         // https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#suffering-from-a-type-mismatch