Don't break a flex line on floating-point rounding noise

tests/YGFlexWrapRoundingTest.cpp

@@ -0,0 +1,86 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#include <gtest/gtest.h>
+#include <yoga/Yoga.h>
+
+namespace {
+
+// Reports the fractional width supplied through the node's context, standing in
+// for a text measurement whose result is not representable on the pixel grid.
+YGSize measureFractionalWidth(
+    YGNodeConstRef node,
+    float /*width*/,
+    YGMeasureMode /*widthMode*/,
+    float /*height*/,
+    YGMeasureMode /*heightMode*/) {
+  const float* measuredWidth =
+      static_cast<const float*>(YGNodeGetContext(const_cast<YGNodeRef>(node)));
+  return YGSize{*measuredWidth, 20.0f};
+}
+
+} // namespace
+
+// A content-sized wrapping row sizes itself to the max-content sum of its
+// items, then recovers the space available to those items by subtracting its
+// padding and border back off. When a measure function contributes a fractional
+// width, that float32 round-trip can land a fraction of an ulp below the sum
+// it came from, while the measurement cache still returns the max-content
+// measurement unchanged because it treats the difference as equal
+// (oldSizeIsMaxContentAndStillFits). The line must not break in that case: the
+// container has already been sized for a single line, so breaking leaves its
+// items laid out on two lines inside a box only tall enough for one.
+//
+// The widths swept here put the row's content sum just under 128 and its outer
+// width just over it, so adding padding and border crosses a float32 binade and
+// the subtraction cannot always recover the original value.
+TEST(YogaTest, content_sized_wrap_row_does_not_break_line_on_rounding) {
+  for (int step = 0; step < 100; step++) {
+    float measuredWidth = 84.0f + static_cast<float>(step) / 100.0f;
+    SCOPED_TRACE(
+        "measured width " + std::to_string(measuredWidth) + " (step " +
+        std::to_string(step) + ")");
+
+    YGNodeRef root = YGNodeNew();
+    YGNodeStyleSetWidth(root, 400.0f);
+    YGNodeStyleSetHeight(root, 400.0f);
+
+    // Shrink-to-fit in the cross axis, so its width comes from its content.
+    YGNodeRef wrapper = YGNodeNew();
+    YGNodeStyleSetAlignSelf(wrapper, YGAlignFlexStart);
+    YGNodeStyleSetBorder(wrapper, YGEdgeAll, 1.0f);
+    YGNodeInsertChild(root, wrapper, 0);
+
+    YGNodeRef row = YGNodeNew();
+    YGNodeStyleSetFlexDirection(row, YGFlexDirectionRow);
+    YGNodeStyleSetFlexWrap(row, YGWrapWrap);
+    YGNodeStyleSetPadding(row, YGEdgeAll, 16.0f);
+    YGNodeInsertChild(wrapper, row, 0);
+
+    YGNodeRef measured = YGNodeNew();
+    YGNodeSetContext(measured, &measuredWidth);
+    YGNodeSetMeasureFunc(measured, measureFractionalWidth);
+    YGNodeInsertChild(row, measured, 0);
+
+    YGNodeRef rigid = YGNodeNew();
+    YGNodeStyleSetWidth(rigid, 20.0f);
+    YGNodeStyleSetHeight(rigid, 20.0f);
+    YGNodeInsertChild(row, rigid, 1);
+
+    YGNodeCalculateLayout(root, 400.0f, 400.0f, YGDirectionLTR);
+
+    // Both items fit on one line by construction: the row is content-sized and
+    // the root is far wider than the content needs.
+    EXPECT_EQ(YGNodeLayoutGetTop(rigid), YGNodeLayoutGetTop(measured));
+
+    // A single line of 20pt content plus 16pt of padding on each side. If the
+    // line broke, the items occupy two lines while the box keeps this height.
+    EXPECT_EQ(52.0f, YGNodeLayoutGetHeight(row));
+
+    YGNodeFreeRecursive(root);
+  }
+}

yoga/algorithm/FlexLine.cpp

@@ -10,6 +10,7 @@
 #include <yoga/algorithm/BoundAxis.h>
 #include <yoga/algorithm/FlexDirection.h>
 #include <yoga/algorithm/FlexLine.h>
+#include <yoga/numeric/Comparison.h>
 
 namespace facebook::yoga {
 
@@ -74,12 +75,26 @@ FlexLine calculateFlexLine(
             ownerWidth)
             .unwrap();
 
+    const float requiredMainDim = sizeConsumedIncludingMinConstraint +
+        flexBasisWithMinAndMaxConstraints + childMarginMainAxis +
+        childLeadingGapMainAxis;
+
     // If this is a multi-line flow and this item pushes us over the available
     // size, we've hit the end of the current line. Break out of the loop and
     // lay out the current line.
-    if (sizeConsumedIncludingMinConstraint + flexBasisWithMinAndMaxConstraints +
-                childMarginMainAxis + childLeadingGapMainAxis >
-            availableInnerMainDim &&
+    //
+    // The overflow test is tolerant of the same epsilon the measurement cache
+    // uses when it decides a cached measurement "still fits" (see
+    // oldSizeIsMaxContentAndStillFits in Cache.cpp). A content-sized wrap
+    // container adds padding and border to the max-content sum to size itself
+    // and then subtracts them again to recover availableInnerMainDim, and in
+    // float32 that round-trip can land a few ulps low. The cache accepts a
+    // basis up to the epsilon wider than the space now available and hands
+    // back the max-content measurement unchanged; without the same tolerance
+    // here, that basis reads as overflow and the line breaks, leaving the
+    // container sized for one line with its items laid out on two.
+    if (requiredMainDim > availableInnerMainDim &&
+        !yoga::inexactEquals(requiredMainDim, availableInnerMainDim) &&
         isNodeFlexWrap && !itemsInFlow.empty()) {
       break;
     }