Fixup hack for flex line size calculation (facebook#39433)

Summary:
X-link: facebook/yoga#1380


Back when rolling out flex gap, we encountered a bug where gap was added to the end of the main axis when a size was not specified.

During flex line justification/sizing, we calculate the amount of space that should be in between children. We erroneously add this, even after the last child element.

For `justify-content`, this space between children is derived from free space along the axis. The only time we have free space is if we had a dimension/dimension constraint already set on the parent. In this case, the extra space added to the end of the flex line is usually never noticed, because we bound `maxLineMainDim` to container dimension constraints at the end of layout, and the error doesn't effect how any children are positioned or sized.

There was at least one screenshot test where this issue showed up though, and I was able to add a slightly different repro where we may have free space without a definite dimension by enforcing a min dimension and not stretching.

{F1091401183}

The new reference is correct, and looking back at diffs, is what this seemed to originally look like when added three years ago. Seems like there may have been a potential regression, but I didn't spot anything suspicious when I looked around the code history.


`betweenMainDim` may still be set for `gap` even if we don't have a sized parent, which makes the extra space propagated to `maxLineMainDim` effect parent size.

Because we were in a code freeze, I opted to have us go with a solution just effecting flex gap, instead of the right one, in case there were any side effects. This cleans up the code to use the right calculation everywhere, and fixes a separate bug, where `endOfLineIndex` and `startOfLineIndex` may not be the last/first in the line if they are out of the layout flow (absolutely positioned, or display: none_

See the original conversation on facebook/yoga#1188

Reviewed By: javache

Differential Revision: D49260049

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

@@ -1306,11 +1306,6 @@ static void YGJustifyMainAxis(
     const auto child = node->getChild(i);
     const Style& childStyle = child->getStyle();
     const LayoutResults& childLayout = child->getLayout();
-    const bool isLastChild = i == flexLine.endOfLineIndex - 1;
-    // remove the gap if it is the last element of the line
-    if (isLastChild) {
-      betweenMainDim -= gap;
-    }
     if (childStyle.display() == YGDisplayNone) {
       continue;
     }
@@ -1344,6 +1339,10 @@ static void YGJustifyMainAxis(
               leadingEdge(mainAxis));
         }
 
+        if (child != flexLine.itemsInFlow.back()) {
+          flexLine.layout.mainDim += betweenMainDim;
+        }
+
         if (child->marginTrailingValue(mainAxis).unit == YGUnitAuto) {
           flexLine.layout.mainDim += flexLine.layout.remainingFreeSpace /
               static_cast<float>(numberOfAutoMarginsOnCurrentLine);
@@ -1354,14 +1353,14 @@ static void YGJustifyMainAxis(
           // If we skipped the flex step, then we can't rely on the measuredDims
           // because they weren't computed. This means we can't call
           // dimensionWithMargin.
-          flexLine.layout.mainDim += betweenMainDim +
+          flexLine.layout.mainDim +=
               child->getMarginForAxis(mainAxis, availableInnerWidth).unwrap() +
               childLayout.computedFlexBasis.unwrap();
           flexLine.layout.crossDim = availableInnerCrossDim;
         } else {
           // The main dimension is the sum of all the elements dimension plus
           // the spacing.
-          flexLine.layout.mainDim += betweenMainDim +
+          flexLine.layout.mainDim +=
               dimensionWithMargin(child, mainAxis, availableInnerWidth);
 
           if (isNodeBaselineLayout) {