Don't break a flex line on floating-point rounding noise (#2022)

Summary:
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 (calculateAvailableInnerDimension). In float32
that add-then-subtract round-trip can land a fraction of an ulp below the
sum it came from.

For an item with a measure function the measurement cache still returns the
max-content measurement in that situation: oldSizeIsMaxContentAndStillFits
accepts a cached width up to the inexactEquals epsilon (1e-4) wider than the
space now available. calculateFlexLine then compared that basis against
availableInnerMainDim with a bare `>`, so the basis the cache had just
accepted as fitting read as overflow and the line broke.

Because the container had already been sized for a single line, the result
was a box one line tall with its items laid out on two, overflowing its own
padding and border. In React Native this reproduced on both iOS and Android
and was extremely sensitive to the measured text width, since only some
values leave a residue inside the window.

Make the overflow test tolerant of the same epsilon the cache uses, so the
two agree. Items without a measure function are unaffected: they take their
exact style width in both passes and never enter that cache path.

The regression test sweeps fractional measured widths whose content sum sits
just under 128 while the outer width sits just over it, so adding padding and
border crosses a float32 binade and the subtraction cannot always recover the
original value.

Changelog: [Internal]

Reviewed By: javache

Differential Revision: D126040642

fbshipit-source-id: 676a2623d5a7efc3a54fe74e443bf75063d57711

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: 2 people

packages/react-native/ReactCommon/yoga/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;
     }