Spec-correct CSS Flexbox §4.5 auto-min-size opt-in on YGConfig (#1966)

Summary:

X-link: facebook/react-native#57015

Implements CSS Flexbox §4.5 automatic minimum sizing in Yoga. When opted in on a `YGConfig`, every flex item whose main-axis `min-{width,height}` is undefined receives a content-derived floor — `min(min-content, specified-size, max-size)` plus any aspect-ratio-transferred lower bound — so it cannot shrink below the size CSS browsers would honor.

How to opt in: clear the new `YGErrataMinSizeUndefinedInsteadOfAuto` errata bit on the config. Default configs carry the bit (preserves today's behaviour); existing trees see no change. The bit is added to `YGErrataClassic` so consumers using that constant continue to get the same default.

Three ways for a node to declare its min-content size along an axis, in this precedence order:

1. **Static per-axis value** via `YGNodeSetMinContentWidth/Height(node, value)`. Pass `YGUndefined` to clear. When set, the §4.5 probe short-circuits at this node — skipping both the dynamic callback and any container recursion. Use for known constants (e.g., images returning 0 per CSS-Images; scroll containers returning 0 on their scroll axis).

2. **Dynamic callback** via `YGNodeSetMinContentMeasureFunc(node, fn)`. Mirrors `YGMeasureFunc`'s shape. The algorithm invokes it for measure-func leaves when no static value is set.

3. **Default fallback**: the existing `YGMeasureFunc` invoked with `AtMost 0`, which text measurers naturally answer with their longest-word width.

For container nodes without a static value, the algorithm walks children directly — sum on the container's own main axis, max on the cross axis — mirroring RenderCore FlexLayout's `computeMinContentSize`. This replaces the previous `calculateLayoutInternal(performLayout=false)` re-entry and skips cross-axis sizing, padding/border resolution, alignment, and baseline machinery that the probe doesn't need.

Per-item opt-outs follow the CSS spec: explicit `min-{width,height}` (including 0), `display: none`, and `overflow != visible`.

Cost when off (default): a single errata-bit check per flex line plus one `FloatOptional` short-circuit per `boundAxisWithAutoMin` call. Sub-microsecond.

Cost when on: one min-content probe per opted-in flex item per layout pass. Static values short-circuit in ~30 ns; measure-func leaves cost one extra `measure()` call; container subtrees do one recursive sum/max walk over in-flow descendants.

Reference: https://www.w3.org/TR/css-flexbox-1/#min-size-auto. Modeled after `xplat/flexlayout/`'s `MinSizeUndefinedInsteadOfAuto` errata path.

Changelog:
[General][Added] - Add CSS Flexbox §4.5 automatic minimum sizing. Opt in by clearing the new `YGErrataMinSizeUndefinedInsteadOfAuto` errata bit on `YGConfig`. Adds `YGNodeSetMinContentWidth/Height` for static contributions and `YGMinContentMeasureFunc` for dynamic ones.'

GH pull request
facebook/react-native#57015
#1966

Reviewed By: fbcbl

Differential Revision: D105720159

Author: adityasharat

enums.py

@@ -86,6 +86,12 @@
         # Absolute nodes will resolve percentages against the inner size of
         # their containing node, not the padding box
         ("AbsolutePercentAgainstInnerSize", 1 << 2),
+        # Treat main-axis `min-{width,height}: undefined` as "no floor"
+        # instead of the CSS §4.5 automatic minimum (which derives a
+        # content-based floor from the item's min-content size). Set by
+        # default on new configs to preserve pre-§4.5 Yoga shrink behavior.
+        # Clear this bit to opt into the spec-correct CSS §4.5 floor.
+        ("MinSizeUndefinedInsteadOfAuto", 1 << 3),
         # Enable all incorrect behavior (preserve compatibility)
         ("All", 0x7FFFFFFF),
         # Enable all errata except for "StretchFlexBasis" (Defaults behavior

java/com/facebook/yoga/YogaConfig.kt

@@ -19,6 +19,14 @@ public abstract class YogaConfig {
 
   public abstract fun setErrata(errata: YogaErrata)
 
+  /**
+   * Sets the errata bitmask directly from an [Int]. Use this when combining multiple [YogaErrata]
+   * values (e.g., `YogaErrata.CLASSIC.intValue() and
+   * YogaErrata.STRETCH_FLEX_BASIS.intValue().inv()`) — the [YogaErrata] enum cannot represent
+   * arbitrary bitmask combinations.
+   */
+  public abstract fun setErrata(errata: Int)
+
   public abstract fun getErrata(): YogaErrata
 
   public abstract fun setLogger(logger: YogaLogger?)

java/com/facebook/yoga/YogaConfigJNIBase.kt

@@ -50,6 +50,10 @@ private constructor(@JvmField protected var nativePointer: Long) : YogaConfig()
     YogaNative.jni_YGConfigSetErrataJNI(nativePointer, errata.intValue())
   }
 
+  public override fun setErrata(errata: Int) {
+    YogaNative.jni_YGConfigSetErrataJNI(nativePointer, errata)
+  }
+
   public override fun getErrata(): YogaErrata =
       YogaErrata.fromInt(YogaNative.jni_YGConfigGetErrataJNI(nativePointer))
 

java/com/facebook/yoga/YogaErrata.kt

@@ -14,6 +14,7 @@ public enum class YogaErrata(public val intValue: Int) {
   STRETCH_FLEX_BASIS(1),
   ABSOLUTE_POSITION_WITHOUT_INSETS_EXCLUDES_PADDING(2),
   ABSOLUTE_PERCENT_AGAINST_INNER_SIZE(4),
+  MIN_SIZE_UNDEFINED_INSTEAD_OF_AUTO(8),
   ALL(2147483647),
   CLASSIC(2147483646);
 
@@ -27,6 +28,7 @@ public enum class YogaErrata(public val intValue: Int) {
           1 -> STRETCH_FLEX_BASIS
           2 -> ABSOLUTE_POSITION_WITHOUT_INSETS_EXCLUDES_PADDING
           4 -> ABSOLUTE_PERCENT_AGAINST_INNER_SIZE
+          8 -> MIN_SIZE_UNDEFINED_INSTEAD_OF_AUTO
           2147483647 -> ALL
           2147483646 -> CLASSIC
           else -> throw IllegalArgumentException("Unknown enum value: $value")

java/com/facebook/yoga/YogaNative.kt

@@ -311,6 +311,22 @@ public object YogaNative {
   @JvmStatic
   public external fun jni_YGNodeSetHasMeasureFuncJNI(nativePointer: Long, hasMeasureFunc: Boolean)
 
+  @JvmStatic
+  public external fun jni_YGNodeSetHasMinContentMeasureFuncJNI(
+      nativePointer: Long,
+      hasMinContentMeasureFunc: Boolean,
+  )
+
+  @JvmStatic
+  public external fun jni_YGNodeSetMinContentWidthJNI(nativePointer: Long, minContentWidth: Float)
+
+  @JvmStatic
+  public external fun jni_YGNodeSetMinContentHeightJNI(nativePointer: Long, minContentHeight: Float)
+
+  @JvmStatic public external fun jni_YGNodeGetMinContentWidthJNI(nativePointer: Long): Float
+
+  @JvmStatic public external fun jni_YGNodeGetMinContentHeightJNI(nativePointer: Long): Float
+
   @JvmStatic
   public external fun jni_YGNodeSetHasBaselineFuncJNI(nativePointer: Long, hasMeasureFunc: Boolean)
 

java/com/facebook/yoga/YogaNode.kt

@@ -228,6 +228,16 @@ public abstract class YogaNode : YogaProps {
 
   abstract override fun setMeasureFunction(measureFunction: YogaMeasureFunction?)
 
+  abstract override fun setMinContentMeasureFunction(measureFunction: YogaMeasureFunction?)
+
+  abstract override fun setMinContentWidth(minContentWidth: Float)
+
+  abstract override fun setMinContentHeight(minContentHeight: Float)
+
+  abstract override fun getMinContentWidth(): Float
+
+  abstract override fun getMinContentHeight(): Float
+
   abstract override fun setBaselineFunction(yogaBaselineFunction: YogaBaselineFunction?)
 
   public abstract val isMeasureDefined: Boolean

java/com/facebook/yoga/YogaNodeJNIBase.kt

@@ -16,6 +16,7 @@ public abstract class YogaNodeJNIBase : YogaNode, Cloneable {
   private var config: YogaConfig? = null
   private var children: MutableList<YogaNodeJNIBase>? = null
   private var measureFunction: YogaMeasureFunction? = null
+  private var minContentMeasureFunction: YogaMeasureFunction? = null
   private var baselineFunction: YogaBaselineFunction? = null
   protected var nativePointer: Long = 0
 
@@ -46,6 +47,7 @@ public abstract class YogaNodeJNIBase : YogaNode, Cloneable {
 
   override fun reset() {
     measureFunction = null
+    minContentMeasureFunction = null
     baselineFunction = null
     data = null
     arr = null
@@ -524,6 +526,25 @@ public abstract class YogaNodeJNIBase : YogaNode, Cloneable {
     YogaNative.jni_YGNodeSetHasMeasureFuncJNI(nativePointer, measureFunction != null)
   }
 
+  override fun setMinContentMeasureFunction(measureFunction: YogaMeasureFunction?) {
+    this.minContentMeasureFunction = measureFunction
+    YogaNative.jni_YGNodeSetHasMinContentMeasureFuncJNI(nativePointer, measureFunction != null)
+  }
+
+  override fun setMinContentWidth(minContentWidth: Float) {
+    YogaNative.jni_YGNodeSetMinContentWidthJNI(nativePointer, minContentWidth)
+  }
+
+  override fun setMinContentHeight(minContentHeight: Float) {
+    YogaNative.jni_YGNodeSetMinContentHeightJNI(nativePointer, minContentHeight)
+  }
+
+  override fun getMinContentWidth(): Float =
+      YogaNative.jni_YGNodeGetMinContentWidthJNI(nativePointer)
+
+  override fun getMinContentHeight(): Float =
+      YogaNative.jni_YGNodeGetMinContentHeightJNI(nativePointer)
+
   override fun setAlwaysFormsContainingBlock(alwaysFormsContainingBlock: Boolean) {
     YogaNative.jni_YGNodeSetAlwaysFormsContainingBlockJNI(
         nativePointer,
@@ -547,6 +568,27 @@ public abstract class YogaNodeJNIBase : YogaNode, Cloneable {
     )
   }
 
+  // Native callback invoked by Yoga during the CSS Flexbox §4.5 auto-min
+  // probe when a min-content measure function is registered. Mirrors
+  // [measure]; see that method's note on non-overridability.
+  @DoNotStrip
+  public fun measureMinContent(
+      width: Float,
+      widthMode: Int,
+      height: Float,
+      heightMode: Int,
+  ): Long {
+    val mf =
+        checkNotNull(minContentMeasureFunction) { "Min-content measure function isn't defined!" }
+    return mf.measure(
+        this,
+        width,
+        YogaMeasureMode.fromInt(widthMode),
+        height,
+        YogaMeasureMode.fromInt(heightMode),
+    )
+  }
+
   override fun setBaselineFunction(yogaBaselineFunction: YogaBaselineFunction?) {
     baselineFunction = yogaBaselineFunction
     YogaNative.jni_YGNodeSetHasBaselineFuncJNI(nativePointer, yogaBaselineFunction != null)

java/com/facebook/yoga/YogaProps.kt

@@ -125,6 +125,28 @@ public interface YogaProps {
 
   public fun setMeasureFunction(measureFunction: YogaMeasureFunction?)
 
+  public fun setMinContentMeasureFunction(measureFunction: YogaMeasureFunction?)
+
+  /**
+   * Sets the static min-content width used by the CSS Flexbox §4.5 automatic minimum sizing probe.
+   * Pass `YogaConstants.UNDEFINED` to clear. See `YGNodeSetMinContentWidth` in the Yoga C API for
+   * full precedence rules.
+   */
+  public fun setMinContentWidth(minContentWidth: Float)
+
+  /**
+   * Sets the static min-content height used by the CSS Flexbox §4.5 automatic minimum sizing probe.
+   * Pass `YogaConstants.UNDEFINED` to clear. See `YGNodeSetMinContentHeight` in the Yoga C API for
+   * full precedence rules.
+   */
+  public fun setMinContentHeight(minContentHeight: Float)
+
+  /** Returns the static min-content width, or `YogaConstants.UNDEFINED` if not set. */
+  public fun getMinContentWidth(): Float
+
+  /** Returns the static min-content height, or `YogaConstants.UNDEFINED` if not set. */
+  public fun getMinContentHeight(): Float
+
   public fun setBaselineFunction(yogaBaselineFunction: YogaBaselineFunction?)
 
   /* Mutable properties - getter and setter with matching types */

java/jni/YGJNIVanilla.cpp

@@ -683,6 +683,83 @@ static void jni_YGNodeSetHasMeasureFuncJNI(
       static_cast<bool>(hasMeasureFunc) ? YGJNIMeasureFunc : nullptr);
 }
 
+static YGSize YGJNIMinContentMeasureFunc(
+    YGNodeConstRef node,
+    float width,
+    YGMeasureMode widthMode,
+    float height,
+    YGMeasureMode heightMode) {
+  if (auto obj = YGNodeJobject(node)) {
+    YGTransferLayoutDirection(node, obj.get());
+    JNIEnv* env = getCurrentEnv();
+    auto objectClass = facebook::yoga::vanillajni::make_local_ref(
+        env, env->GetObjectClass(obj.get()));
+    // NOLINTNEXTLINE(misc-misplaced-const)
+    static const jmethodID methodId = facebook::yoga::vanillajni::getMethodId(
+        env, objectClass.get(), "measureMinContent", "(FIFI)J");
+    // NOLINTNEXTLINE(cppcoreguidelines-pro-type-vararg)
+    const auto measureResult = facebook::yoga::vanillajni::callLongMethod(
+        env, obj.get(), methodId, width, widthMode, height, heightMode);
+
+    uint32_t wBits = 0xFFFFFFFF & (measureResult >> 32);
+    uint32_t hBits = 0xFFFFFFFF & measureResult;
+    auto measuredWidth = std::bit_cast<float>(wBits);
+    auto measuredHeight = std::bit_cast<float>(hBits);
+
+    return YGSize{measuredWidth, measuredHeight};
+  } else {
+    return YGSize{
+        widthMode == YGMeasureModeUndefined ? 0 : width,
+        heightMode == YGMeasureModeUndefined ? 0 : height,
+    };
+  }
+}
+
+static void jni_YGNodeSetHasMinContentMeasureFuncJNI(
+    JNIEnv* /*env*/,
+    jobject /*obj*/,
+    jlong nativePointer,
+    jboolean hasMinContentMeasureFunc) {
+  YGNodeSetMinContentMeasureFunc(
+      _jlong2YGNodeRef(nativePointer),
+      static_cast<bool>(hasMinContentMeasureFunc) ? YGJNIMinContentMeasureFunc
+                                                  : nullptr);
+}
+
+static void jni_YGNodeSetMinContentWidthJNI(
+    JNIEnv* /*env*/,
+    jobject /*obj*/,
+    jlong nativePointer,
+    jfloat minContentWidth) {
+  YGNodeSetMinContentWidth(
+      _jlong2YGNodeRef(nativePointer), static_cast<float>(minContentWidth));
+}
+
+static void jni_YGNodeSetMinContentHeightJNI(
+    JNIEnv* /*env*/,
+    jobject /*obj*/,
+    jlong nativePointer,
+    jfloat minContentHeight) {
+  YGNodeSetMinContentHeight(
+      _jlong2YGNodeRef(nativePointer), static_cast<float>(minContentHeight));
+}
+
+static jfloat jni_YGNodeGetMinContentWidthJNI(
+    JNIEnv* /*env*/,
+    jobject /*obj*/,
+    jlong nativePointer) {
+  return static_cast<jfloat>(
+      YGNodeGetMinContentWidth(_jlong2YGNodeRef(nativePointer)));
+}
+
+static jfloat jni_YGNodeGetMinContentHeightJNI(
+    JNIEnv* /*env*/,
+    jobject /*obj*/,
+    jlong nativePointer) {
+  return static_cast<jfloat>(
+      YGNodeGetMinContentHeight(_jlong2YGNodeRef(nativePointer)));
+}
+
 static float YGJNIBaselineFunc(YGNodeConstRef node, float width, float height) {
   if (auto obj = YGNodeJobject(node)) {
     JNIEnv* env = getCurrentEnv();
@@ -1058,6 +1135,23 @@ static JNINativeMethod methods[] = {
     {"jni_YGNodeSetHasMeasureFuncJNI",
      "(JZ)V",
      (void*)jni_YGNodeSetHasMeasureFuncJNI},
+    // NOLINTBEGIN(cppcoreguidelines-pro-type-cstyle-cast)
+    {"jni_YGNodeSetHasMinContentMeasureFuncJNI",
+     "(JZ)V",
+     (void*)jni_YGNodeSetHasMinContentMeasureFuncJNI},
+    {"jni_YGNodeSetMinContentWidthJNI",
+     "(JF)V",
+     (void*)jni_YGNodeSetMinContentWidthJNI},
+    {"jni_YGNodeSetMinContentHeightJNI",
+     "(JF)V",
+     (void*)jni_YGNodeSetMinContentHeightJNI},
+    {"jni_YGNodeGetMinContentWidthJNI",
+     "(J)F",
+     (void*)jni_YGNodeGetMinContentWidthJNI},
+    {"jni_YGNodeGetMinContentHeightJNI",
+     "(J)F",
+     (void*)jni_YGNodeGetMinContentHeightJNI},
+    // NOLINTEND(cppcoreguidelines-pro-type-cstyle-cast)
     {"jni_YGNodeStyleGetGapJNI", "(JI)J", (void*)jni_YGNodeStyleGetGapJNI},
     {"jni_YGNodeStyleSetGapJNI", "(JIF)V", (void*)jni_YGNodeStyleSetGapJNI},
     {"jni_YGNodeStyleSetGapPercentJNI",

javascript/src/generated/YGEnums.ts

@@ -61,6 +61,7 @@ export enum Errata {
   StretchFlexBasis = 1,
   AbsolutePositionWithoutInsetsExcludesPadding = 2,
   AbsolutePercentAgainstInnerSize = 4,
+  MinSizeUndefinedInsteadOfAuto = 8,
   All = 2147483647,
   Classic = 2147483646,
 }
@@ -188,6 +189,7 @@ const constants = {
   ERRATA_STRETCH_FLEX_BASIS: Errata.StretchFlexBasis,
   ERRATA_ABSOLUTE_POSITION_WITHOUT_INSETS_EXCLUDES_PADDING: Errata.AbsolutePositionWithoutInsetsExcludesPadding,
   ERRATA_ABSOLUTE_PERCENT_AGAINST_INNER_SIZE: Errata.AbsolutePercentAgainstInnerSize,
+  ERRATA_MIN_SIZE_UNDEFINED_INSTEAD_OF_AUTO: Errata.MinSizeUndefinedInsteadOfAuto,
   ERRATA_ALL: Errata.All,
   ERRATA_CLASSIC: Errata.Classic,
   EXPERIMENTAL_FEATURE_WEB_FLEX_BASIS: ExperimentalFeature.WebFlexBasis,