Re-land: fixYogaFlexBasisFitContentInMainAxis (#1917)

Summary:
X-link: react/react-native#56064

Pull Request resolved: #1917

Re-land of D94658492 with fixes, which was reverted in D95669495.

## Context

D94658492 added the `fixYogaFlexBasisFitContentInMainAxis` flag to avoid unnecessary re-measurement cascades in Yoga. When Yoga computes flex basis for container children, the legacy behavior applies a `FitContent` constraint in the main axis, bounding the child's measurement by the parent's available space. This creates a dependency between the child's flex basis and the parent's content-determined size — when one sibling changes size, all siblings get re-measured and their shadow nodes get cloned unnecessarily.

The fix switches from `FitContent` to `MaxContent` for non-measure container children under auto-height parents, making each child's flex basis independent of the parent's size.

## What went wrong

D94658492 modeled the fix as a `YogaErrata` bit (`FLEX_BASIS_FIT_CONTENT_IN_MAIN_AXIS`). Errata flags are bitmasks, and apps that opt into `ALL` or `CLASSIC` errata (like IGVR and Airwave) inadvertently picked up the new behavior without explicitly enabling the feature flag, causing breakages.

## What changed in this re-land

This diff models the fix as a `YogaExperimentalFeature` (`FIX_FLEX_BASIS_FIT_CONTENT`) instead of a `YogaErrata` bit. Experimental features are individually opt-in, so existing apps won't accidentally pick up the change.

This diff only wires up the RN feature flag infrastructure (flag defaults to `false`). The iOS MobileConfig override and the Yoga layout logic will be landed in follow-up diffs.

changelog: [internal]

Reviewed By: javache, NickGerleman

Differential Revision: D95852922

fbshipit-source-id: e2b1aa7a5d8f340011123679f721c62396b5caa5

Author: sammy-SC

enums.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python3
+#!/usr/bin/env fbpython
 # Copyright (c) Meta Platforms, Inc. and affiliates.
 #
 # This source code is licensed under the MIT license found in the
@@ -66,6 +66,9 @@
     "ExperimentalFeature": [
         # Mimic web flex-basis behavior (experiment may be broken)
         "WebFlexBasis",
+        # Fix flex basis computation to not apply FitContent constraint in the
+        # main axis for non-measure container nodes
+        "FixFlexBasisFitContent",
     ],
     "Gutter": ["Column", "Row", "All"],
     "GridTrackType": ["Auto", "Points", "Percent", "Fr", "Minmax"],

gentest/fixtures/YGFlexBasisFitContentTest.html

@@ -0,0 +1,68 @@
+<!-- Container child with content overflowing definite column parent.
+     Flex basis uses content size regardless of ExperimentalFeature state,
+     because FitContent and MaxContent produce the same result for containers. -->
+<div id="container_child_overflows_definite_parent_column"
+     style="width: 200px; height: 300px;">
+  <div>
+    <div style="height: 500px; width: 50px;"></div>
+  </div>
+</div>
+
+<!-- Row variant: container child overflows definite row parent. -->
+<div id="container_child_overflows_definite_parent_row"
+     style="width: 300px; height: 200px; flex-direction: row;">
+  <div>
+    <div style="width: 500px; height: 50px;"></div>
+  </div>
+</div>
+
+<!-- Container child content fits within parent bounds.
+     Same behavior with and without feature (no capping needed). -->
+<div id="container_child_within_bounds_column"
+     style="width: 200px; height: 300px;">
+  <div>
+    <div style="height: 100px; width: 50px;"></div>
+  </div>
+</div>
+
+<!-- Multiple container children, both overflowing definite column parent. -->
+<div id="multiple_container_children_overflow_column"
+     style="width: 200px; height: 300px;">
+  <div>
+    <div style="height: 400px;"></div>
+  </div>
+  <div>
+    <div style="height: 500px;"></div>
+  </div>
+</div>
+
+<!-- Scroll container: children always use MaxContent in main axis.
+     Same behavior with and without feature. -->
+<div id="scroll_container_column"
+     style="width: 200px; height: 300px; overflow: scroll;">
+  <div>
+    <div style="height: 500px;"></div>
+  </div>
+</div>
+
+<!-- Mix of explicit-height child and overflowing container child. -->
+<div id="explicit_and_container_children_column"
+     style="width: 200px; height: 300px;">
+  <div style="height: 100px;"></div>
+  <div>
+    <div style="height: 500px;"></div>
+  </div>
+</div>
+
+<!-- Items with flex-basis inside a scroll container's auto-height
+     content container. With FixFlexBasisFitContent, flex-basis is
+     respected even when the content container's main axis size is
+     indefinite (NaN). Without the feature, flex-basis would be ignored. -->
+<div id="flex_basis_in_scroll_content_container"
+     data-experiments="FixFlexBasisFitContent"
+     style="width: 200px; height: 300px; overflow: scroll;">
+  <div>
+    <div style="flex-basis: 200px;"></div>
+    <div style="flex-basis: 300px;"></div>
+  </div>
+</div>

java/com/facebook/yoga/YogaExperimentalFeature.java

@@ -10,7 +10,8 @@
 package com.facebook.yoga;
 
 public enum YogaExperimentalFeature {
-  WEB_FLEX_BASIS(0);
+  WEB_FLEX_BASIS(0),
+  FIX_FLEX_BASIS_FIT_CONTENT(1);
 
   private final int mIntValue;
 
@@ -25,6 +26,7 @@ public int intValue() {
   public static YogaExperimentalFeature fromInt(int value) {
     switch (value) {
       case 0: return WEB_FLEX_BASIS;
+      case 1: return FIX_FLEX_BASIS_FIT_CONTENT;
       default: throw new IllegalArgumentException("Unknown enum value: " + value);
     }
   }