Fixup margin: auto and justification behavior for overflowed containers

Summary:
Fixes facebook/yoga#978

1. Don't allow auto margin spaces to become a negative length
2. Replicate fallback alignment behavior specified by box-alignment spec that we are using for align-content.

Differential Revision: D56091577

packages/react-native/ReactCommon/yoga/yoga/algorithm/Align.h

@@ -26,4 +26,47 @@ inline Align resolveChildAlignment(
   return align;
 }
 
+/**
+ * Fallback alignment to use on overflow
+ * https://www.w3.org/TR/css-align-3/#distribution-values
+ */
+constexpr Align fallbackAlignment(Align align) {
+  switch (align) {
+      // Fallback to flex-start
+    case Align::SpaceBetween:
+    case Align::Stretch:
+      return Align::FlexStart;
+
+    // Fallback to safe center. TODO: This should be aligned to Start
+    // instead of FlexStart (for row-reverse containers)
+    case Align::SpaceAround:
+    case Align::SpaceEvenly:
+      return Align::FlexStart;
+    default:
+      return align;
+  }
+}
+
+/**
+ * Fallback alignment to use on overflow
+ * https://www.w3.org/TR/css-align-3/#distribution-values
+ */
+constexpr Justify fallbackAlignment(Justify align) {
+  switch (align) {
+      // Fallback to flex-start
+    case Justify::SpaceBetween:
+      // TODO: Support `justify-content: stretch`
+      // case Justify::Stretch:
+      return Justify::FlexStart;
+
+    // Fallback to safe center. TODO: This should be aligned to Start
+    // instead of FlexStart (for row-reverse containers)
+    case Justify::SpaceAround:
+    case Justify::SpaceEvenly:
+      return Justify::FlexStart;
+    default:
+      return align;
+  }
+}
+
 } // namespace facebook::yoga

packages/react-native/ReactCommon/yoga/yoga/algorithm/CalculateLayout.cpp

@@ -958,27 +958,16 @@ static void justifyMainAxis(
     }
   }
 
-  int numberOfAutoMarginsOnCurrentLine = 0;
-  for (size_t i = startOfLineIndex; i < flexLine.endOfLineIndex; i++) {
-    auto child = node->getChild(i);
-    if (child->style().positionType() != PositionType::Absolute) {
-      if (child->style().flexStartMarginIsAuto(mainAxis, direction)) {
-        numberOfAutoMarginsOnCurrentLine++;
-      }
-      if (child->style().flexEndMarginIsAuto(mainAxis, direction)) {
-        numberOfAutoMarginsOnCurrentLine++;
-      }
-    }
-  }
-
   // In order to position the elements in the main axis, we have two controls.
   // The space between the beginning and the first element and the space between
   // each two elements.
   float leadingMainDim = 0;
   float betweenMainDim = gap;
-  const Justify justifyContent = node->style().justifyContent();
+  const Justify justifyContent = flexLine.layout.remainingFreeSpace >= 0
+      ? node->style().justifyContent()
+      : fallbackAlignment(node->style().justifyContent());
 
-  if (numberOfAutoMarginsOnCurrentLine == 0) {
+  if (flexLine.numberOfAutoMargins == 0) {
     switch (justifyContent) {
       case Justify::Center:
         leadingMainDim = flexLine.layout.remainingFreeSpace / 2;
@@ -988,8 +977,7 @@ static void justifyMainAxis(
         break;
       case Justify::SpaceBetween:
         if (flexLine.itemsInFlow.size() > 1) {
-          betweenMainDim +=
-              yoga::maxOrDefined(flexLine.layout.remainingFreeSpace, 0.0f) /
+          betweenMainDim += flexLine.layout.remainingFreeSpace /
               static_cast<float>(flexLine.itemsInFlow.size() - 1);
         }
         break;
@@ -1042,9 +1030,10 @@ static void justifyMainAxis(
       // We need to do that only for relative elements. Absolute elements do not
       // take part in that phase.
       if (childStyle.positionType() != PositionType::Absolute) {
-        if (child->style().flexStartMarginIsAuto(mainAxis, direction)) {
+        if (child->style().flexStartMarginIsAuto(mainAxis, direction) &&
+            flexLine.layout.remainingFreeSpace > 0.0f) {
           flexLine.layout.mainDim += flexLine.layout.remainingFreeSpace /
-              static_cast<float>(numberOfAutoMarginsOnCurrentLine);
+              static_cast<float>(flexLine.numberOfAutoMargins);
         }
 
         if (performLayout) {
@@ -1058,9 +1047,10 @@ static void justifyMainAxis(
           flexLine.layout.mainDim += betweenMainDim;
         }
 
-        if (child->style().flexEndMarginIsAuto(mainAxis, direction)) {
+        if (child->style().flexEndMarginIsAuto(mainAxis, direction) &&
+            flexLine.layout.remainingFreeSpace > 0.0f) {
           flexLine.layout.mainDim += flexLine.layout.remainingFreeSpace /
-              static_cast<float>(numberOfAutoMarginsOnCurrentLine);
+              static_cast<float>(flexLine.numberOfAutoMargins);
         }
         bool canSkipFlex =
             !performLayout && sizingModeCrossDim == SizingMode::StretchFit;
@@ -1746,27 +1736,11 @@ static void calculateLayoutImpl(
 
     const float remainingAlignContentDim = innerCrossDim - totalLineCrossDim;
 
-    // Apply fallback alignments on overflow
-    // https://www.w3.org/TR/css-align-3/#distribution-values
-    const auto appliedAlignContent =
-        remainingAlignContentDim >= 0 ? node->style().alignContent() : [&]() {
-          switch (node->style().alignContent()) {
-            // Fallback to flex-start
-            case Align::SpaceBetween:
-            case Align::Stretch:
-              return Align::FlexStart;
-
-            // Fallback to safe center. TODO: This should be aligned to Start
-            // instead of FlexStart (for row-reverse containers)
-            case Align::SpaceAround:
-            case Align::SpaceEvenly:
-              return Align::FlexStart;
-            default:
-              return node->style().alignContent();
-          }
-        }();
+    const auto alignContent = remainingAlignContentDim >= 0
+        ? node->style().alignContent()
+        : fallbackAlignment(node->style().alignContent());
 
-    switch (appliedAlignContent) {
+    switch (alignContent) {
       case Align::FlexEnd:
         currentLead += remainingAlignContentDim;
         break;

packages/react-native/ReactCommon/yoga/yoga/algorithm/FlexLine.cpp

@@ -27,6 +27,7 @@ FlexLine calculateFlexLine(
   float sizeConsumed = 0.0f;
   float totalFlexGrowFactors = 0.0f;
   float totalFlexShrinkScaledFactors = 0.0f;
+  size_t numberOfAutoMargins = 0;
   size_t endOfLineIndex = startOfLineIndex;
   size_t firstElementInLineIndex = startOfLineIndex;
 
@@ -48,6 +49,13 @@ FlexLine calculateFlexLine(
       continue;
     }
 
+    if (child->style().flexStartMarginIsAuto(mainAxis, ownerDirection)) {
+      numberOfAutoMargins++;
+    }
+    if (child->style().flexEndMarginIsAuto(mainAxis, ownerDirection)) {
+      numberOfAutoMargins++;
+    }
+
     const bool isFirstElementInLine =
         (endOfLineIndex - firstElementInLineIndex) == 0;
 
@@ -101,10 +109,11 @@ FlexLine calculateFlexLine(
   }
 
   return FlexLine{
-      std::move(itemsInFlow),
-      sizeConsumed,
-      endOfLineIndex,
-      FlexLineRunningLayout{
+      .itemsInFlow = std::move(itemsInFlow),
+      .sizeConsumed = sizeConsumed,
+      .endOfLineIndex = endOfLineIndex,
+      .numberOfAutoMargins = numberOfAutoMargins,
+      .layout = FlexLineRunningLayout{
           totalFlexGrowFactors,
           totalFlexShrinkScaledFactors,
       }};

packages/react-native/ReactCommon/yoga/yoga/algorithm/FlexLine.h

@@ -52,6 +52,9 @@ struct FlexLine {
   // The index of the first item beyond the current line.
   const size_t endOfLineIndex{0};
 
+  // Number of edges along the line flow with an auto margin.
+  const size_t numberOfAutoMargins{0};
+
   // Layout information about the line computed in steps after line-breaking
   FlexLineRunningLayout layout{};
 };