[CSS Math Functions] Correct mod() evaluation

https://bugs.webkit.org/show_bug.cgi?id=259690
rdar://113213059

Reviewed by Simon Fraser.

According to https://drafts.csswg.org/css-values/#round-func :

Their behavior diverges if the A value and the B step are on opposite sides of zero: mod() (short
for “modulus”) continues to choose the integer multiple of B that puts the value between zero and
B, as above (guaranteeing that the result will either be zero or share the sign of B, not A), while
rem() (short for "remainder") chooses the integer multiple of B that puts the value between zero
and -B, avoiding changing the sign of the value.

* LayoutTests/imported/w3c/web-platform-tests/css/css-values/round-mod-rem-computed-expected.txt:
* Source/WebCore/platform/calc/CalcOperator.h:
(WebCore::evaluateCalcExpression):

Canonical link: https://commits.webkit.org/266485@main

LayoutTests/imported/w3c/web-platform-tests/css/css-values/round-mod-rem-computed-expected.txt

@@ -26,7 +26,7 @@ PASS calc(mod(18,5) * 2 + mod(17,5)) should be used-value-equivalent to 8
 PASS calc(rem(mod(18,5),5)) should be used-value-equivalent to 3
 PASS calc(rem(mod(18,5),mod(17,5))) should be used-value-equivalent to 1
 PASS calc(mod(-140,-90)) should be used-value-equivalent to -50
-FAIL calc(mod(rem(1,18)* -1,5)) should be used-value-equivalent to 4 assert_equals: calc(mod(rem(1,18)* -1,5)) and 4 serialize to the same thing in used values. expected "matrix(4, 0, 0, 4, 0, 0)" but got "matrix(-1, 0, 0, -1, 0, 0)"
+PASS calc(mod(rem(1,18)* -1,5)) should be used-value-equivalent to 4
 PASS round(10px,6px) should be used-value-equivalent to 12px
 PASS round(10cm,6cm) should be used-value-equivalent to 12cm
 PASS round(10mm,6mm) should be used-value-equivalent to 12mm

Source/WebCore/platform/calc/CalcOperator.h

@@ -225,11 +225,18 @@ double evaluateCalcExpression(CalcOperator calcOperator, const Vector<T>& childr
             return std::numeric_limits<double>::quiet_NaN();
         auto left = evaluate(children[0]);
         auto right = evaluate(children[1]);
-        if (!right)
+        // In mod(A, B) only, if B is infinite and A has opposite sign to B
+        // (including an oppositely-signed zero), the result is NaN.
+        // https://drafts.csswg.org/css-values/#round-infinities
+        if (std::isinf(right) && std::signbit(left) != std::signbit(right))
             return std::numeric_limits<double>::quiet_NaN();
-        if ((left < 0) == (right < 0))
-            return std::fmod(left, right);
-        return std::remainder(left, right);
+        auto result = std::fmod(left, right);
+        // If the result is on opposite side of zero from B,
+        // put it between 0 and B.
+        // https://drafts.csswg.org/css-values/#round-func
+        if (std::signbit(result) != std::signbit(right))
+            result += right;
+        return result;
     }
     case CalcOperator::Rem: {
         if (children.size() != 2)