Remove legacy absolute positioning path (#1725)

Summary:
X-link: facebook/react-native#46984


The legacy (wrong) absolute positioning path positions in two places, including work that is definitely always overwritten in the new absolute layout path.

This came up before for position: static, but we didn't clean this up at the time. This code is also now leading display: contents impl being more annoying.

This diff tries to converge to the more spec correct implementation of positioning here, that also only happens in one place. 

Previous path would potentially also incorrectly justify when `justify-content` was non-default, but not handled in the previous few cases? We don't have access to the flexLine at this point later, and apart from the existing tests now passing I reused the new correct logic for justification (spec says we should position child as if its the only child in the container https://www.w3.org/TR/css-flexbox-1/#abspos-items).

I added a new, more scoped errata `AbsolutePositionWithoutInsetsExcludesPadding` to preserve some of the legacy behavior that showed as very breaking. 

I also did not try removing `AbsolutePercentAgainstInnerSize` which I suspect would be more breaking than this change.

Changelog: 
[General][Breaking] - More spec compliant absolute positioning

Reviewed By: joevilches

Differential Revision: D64244949

enums.py

@@ -61,9 +61,10 @@
         # Allows main-axis flex basis to be stretched without flexGrow being
         # set (previously referred to as "UseLegacyStretchBehaviour")
         ("StretchFlexBasis", 1 << 0),
-        # Positioning of absolute nodes will have various bugs related to
-        # justification, alignment, and insets
-        ("AbsolutePositioningIncorrect", 1 << 1),
+        # Absolute position in a given axis will be relative to the padding
+        # edge of the parent container instead of the content edge when a
+        # specific inset (top/bottom/left/right) is not set.
+        ("AbsolutePositionWithoutInsetsExcludesPadding", 1 << 1),
         # Absolute nodes will resolve percentages against the inner size of
         # their containing node, not the padding box
         ("AbsolutePercentAgainstInnerSize", 1 << 2),

java/com/facebook/yoga/YogaErrata.java

@@ -12,7 +12,7 @@
 public enum YogaErrata {
   NONE(0),
   STRETCH_FLEX_BASIS(1),
-  ABSOLUTE_POSITIONING_INCORRECT(2),
+  ABSOLUTE_POSITION_WITHOUT_INSETS_EXCLUDES_PADDING(2),
   ABSOLUTE_PERCENT_AGAINST_INNER_SIZE(4),
   ALL(2147483647),
   CLASSIC(2147483646);
@@ -31,7 +31,7 @@ public static YogaErrata fromInt(int value) {
     switch (value) {
       case 0: return NONE;
       case 1: return STRETCH_FLEX_BASIS;
-      case 2: return ABSOLUTE_POSITIONING_INCORRECT;
+      case 2: return ABSOLUTE_POSITION_WITHOUT_INSETS_EXCLUDES_PADDING;
       case 4: return ABSOLUTE_PERCENT_AGAINST_INNER_SIZE;
       case 2147483647: return ALL;
       case 2147483646: return CLASSIC;

javascript/src/generated/YGEnums.ts

@@ -55,7 +55,7 @@ export enum Edge {
 export enum Errata {
   None = 0,
   StretchFlexBasis = 1,
-  AbsolutePositioningIncorrect = 2,
+  AbsolutePositionWithoutInsetsExcludesPadding = 2,
   AbsolutePercentAgainstInnerSize = 4,
   All = 2147483647,
   Classic = 2147483646,
@@ -162,7 +162,7 @@ const constants = {
   EDGE_ALL: Edge.All,
   ERRATA_NONE: Errata.None,
   ERRATA_STRETCH_FLEX_BASIS: Errata.StretchFlexBasis,
-  ERRATA_ABSOLUTE_POSITIONING_INCORRECT: Errata.AbsolutePositioningIncorrect,
+  ERRATA_ABSOLUTE_POSITION_WITHOUT_INSETS_EXCLUDES_PADDING: Errata.AbsolutePositionWithoutInsetsExcludesPadding,
   ERRATA_ABSOLUTE_PERCENT_AGAINST_INNER_SIZE: Errata.AbsolutePercentAgainstInnerSize,
   ERRATA_ALL: Errata.All,
   ERRATA_CLASSIC: Errata.Classic,

yoga/YGEnums.cpp

@@ -105,8 +105,8 @@ const char* YGErrataToString(const YGErrata value) {
       return "none";
     case YGErrataStretchFlexBasis:
       return "stretch-flex-basis";
-    case YGErrataAbsolutePositioningIncorrect:
-      return "absolute-positioning-incorrect";
+    case YGErrataAbsolutePositionWithoutInsetsExcludesPadding:
+      return "absolute-position-without-insets-excludes-padding";
     case YGErrataAbsolutePercentAgainstInnerSize:
       return "absolute-percent-against-inner-size";
     case YGErrataAll:

yoga/YGEnums.h

@@ -61,7 +61,7 @@ YG_ENUM_DECL(
     YGErrata,
     YGErrataNone = 0,
     YGErrataStretchFlexBasis = 1,
-    YGErrataAbsolutePositioningIncorrect = 2,
+    YGErrataAbsolutePositionWithoutInsetsExcludesPadding = 2,
     YGErrataAbsolutePercentAgainstInnerSize = 4,
     YGErrataAll = 2147483647,
     YGErrataClassic = 2147483646)

yoga/algorithm/AbsoluteLayout.cpp

@@ -19,12 +19,15 @@ static inline void setFlexStartLayoutPosition(
     const Direction direction,
     const FlexDirection axis,
     const float containingBlockWidth) {
-  child->setLayoutPosition(
-      child->style().computeFlexStartMargin(
-          axis, direction, containingBlockWidth) +
-          parent->getLayout().border(flexStartEdge(axis)) +
-          parent->getLayout().padding(flexStartEdge(axis)),
-      flexStartEdge(axis));
+  float position = child->style().computeFlexStartMargin(
+                       axis, direction, containingBlockWidth) +
+      parent->getLayout().border(flexStartEdge(axis));
+
+  if (!child->hasErrata(Errata::AbsolutePositionWithoutInsetsExcludesPadding)) {
+    position += parent->getLayout().padding(flexStartEdge(axis));
+  }
+
+  child->setLayoutPosition(position, flexStartEdge(axis));
 }
 
 static inline void setFlexEndLayoutPosition(
@@ -33,15 +36,16 @@ static inline void setFlexEndLayoutPosition(
     const Direction direction,
     const FlexDirection axis,
     const float containingBlockWidth) {
+  float flexEndPosition = parent->getLayout().border(flexEndEdge(axis)) +
+      child->style().computeFlexEndMargin(
+          axis, direction, containingBlockWidth);
+
+  if (!child->hasErrata(Errata::AbsolutePositionWithoutInsetsExcludesPadding)) {
+    flexEndPosition += parent->getLayout().padding(flexEndEdge(axis));
+  }
+
   child->setLayoutPosition(
-      getPositionOfOppositeEdge(
-          parent->getLayout().border(flexEndEdge(axis)) +
-              parent->getLayout().padding(flexEndEdge(axis)) +
-              child->style().computeFlexEndMargin(
-                  axis, direction, containingBlockWidth),
-          axis,
-          parent,
-          child),
+      getPositionOfOppositeEdge(flexEndPosition, axis, parent, child),
       flexStartEdge(axis));
 }
 
@@ -51,22 +55,30 @@ static inline void setCenterLayoutPosition(
     const Direction direction,
     const FlexDirection axis,
     const float containingBlockWidth) {
-  const float parentContentBoxSize =
+  float parentContentBoxSize =
       parent->getLayout().measuredDimension(dimension(axis)) -
       parent->getLayout().border(flexStartEdge(axis)) -
-      parent->getLayout().border(flexEndEdge(axis)) -
-      parent->getLayout().padding(flexStartEdge(axis)) -
-      parent->getLayout().padding(flexEndEdge(axis));
+      parent->getLayout().border(flexEndEdge(axis));
+
+  if (!child->hasErrata(Errata::AbsolutePositionWithoutInsetsExcludesPadding)) {
+    parentContentBoxSize -= parent->getLayout().padding(flexStartEdge(axis));
+    parentContentBoxSize -= parent->getLayout().padding(flexEndEdge(axis));
+  }
+
   const float childOuterSize =
       child->getLayout().measuredDimension(dimension(axis)) +
       child->style().computeMarginForAxis(axis, containingBlockWidth);
-  child->setLayoutPosition(
-      (parentContentBoxSize - childOuterSize) / 2.0f +
-          parent->getLayout().border(flexStartEdge(axis)) +
-          parent->getLayout().padding(flexStartEdge(axis)) +
-          child->style().computeFlexStartMargin(
-              axis, direction, containingBlockWidth),
-      flexStartEdge(axis));
+
+  float position = (parentContentBoxSize - childOuterSize) / 2.0f +
+      parent->getLayout().border(flexStartEdge(axis)) +
+      child->style().computeFlexStartMargin(
+          axis, direction, containingBlockWidth);
+
+  if (!child->hasErrata(Errata::AbsolutePositionWithoutInsetsExcludesPadding)) {
+    position += parent->getLayout().padding(flexStartEdge(axis));
+  }
+
+  child->setLayoutPosition(position, flexStartEdge(axis));
 }
 
 static void justifyAbsoluteChild(
@@ -133,62 +145,6 @@ static void alignAbsoluteChild(
   }
 }
 
-// To ensure no breaking changes, we preserve the legacy way of positioning
-// absolute children and determine if we should use it using an errata.
-static void positionAbsoluteChildLegacy(
-    const yoga::Node* const containingNode,
-    const yoga::Node* const parent,
-    yoga::Node* child,
-    const Direction direction,
-    const FlexDirection axis,
-    const bool isMainAxis,
-    const float containingBlockWidth,
-    const float containingBlockHeight) {
-  const bool isAxisRow = isRow(axis);
-  const bool shouldCenter = isMainAxis
-      ? parent->style().justifyContent() == Justify::Center
-      : resolveChildAlignment(parent, child) == Align::Center;
-  const bool shouldFlexEnd = isMainAxis
-      ? parent->style().justifyContent() == Justify::FlexEnd
-      : ((resolveChildAlignment(parent, child) == Align::FlexEnd) ^
-         (parent->style().flexWrap() == Wrap::WrapReverse));
-
-  if (child->style().isFlexEndPositionDefined(axis, direction) &&
-      (!child->style().isFlexStartPositionDefined(axis, direction) ||
-       child->style().isFlexStartPositionAuto(axis, direction))) {
-    child->setLayoutPosition(
-        containingNode->getLayout().measuredDimension(dimension(axis)) -
-            child->getLayout().measuredDimension(dimension(axis)) -
-            containingNode->style().computeFlexEndBorder(axis, direction) -
-            child->style().computeFlexEndMargin(
-                axis,
-                direction,
-                isAxisRow ? containingBlockWidth : containingBlockHeight) -
-            child->style().computeFlexEndPosition(
-                axis,
-                direction,
-                isAxisRow ? containingBlockWidth : containingBlockHeight),
-        flexStartEdge(axis));
-  } else if (
-      (!child->style().isFlexStartPositionDefined(axis, direction) ||
-       child->style().isFlexStartPositionAuto(axis, direction)) &&
-      shouldCenter) {
-    child->setLayoutPosition(
-        (parent->getLayout().measuredDimension(dimension(axis)) -
-         child->getLayout().measuredDimension(dimension(axis))) /
-            2.0f,
-        flexStartEdge(axis));
-  } else if (
-      (!child->style().isFlexStartPositionDefined(axis, direction) ||
-       child->style().isFlexStartPositionAuto(axis, direction)) &&
-      shouldFlexEnd) {
-    child->setLayoutPosition(
-        (parent->getLayout().measuredDimension(dimension(axis)) -
-         child->getLayout().measuredDimension(dimension(axis))),
-        flexStartEdge(axis));
-  }
-}
-
 /*
  * Absolutely positioned nodes do not participate in flex layout and thus their
  * positions can be determined independently from the rest of their siblings.
@@ -205,7 +161,7 @@ static void positionAbsoluteChildLegacy(
  * This function does that positioning for the given axis. The spec has more
  * information on this topic: https://www.w3.org/TR/css-flexbox-1/#abspos-items
  */
-static void positionAbsoluteChildImpl(
+static void positionAbsoluteChild(
     const yoga::Node* const containingNode,
     const yoga::Node* const parent,
     yoga::Node* child,
@@ -267,36 +223,6 @@ static void positionAbsoluteChildImpl(
   }
 }
 
-static void positionAbsoluteChild(
-    const yoga::Node* const containingNode,
-    const yoga::Node* const parent,
-    yoga::Node* child,
-    const Direction direction,
-    const FlexDirection axis,
-    const bool isMainAxis,
-    const float containingBlockWidth,
-    const float containingBlockHeight) {
-  child->hasErrata(Errata::AbsolutePositioningIncorrect)
-      ? positionAbsoluteChildLegacy(
-            containingNode,
-            parent,
-            child,
-            direction,
-            axis,
-            isMainAxis,
-            containingBlockWidth,
-            containingBlockHeight)
-      : positionAbsoluteChildImpl(
-            containingNode,
-            parent,
-            child,
-            direction,
-            axis,
-            isMainAxis,
-            containingBlockWidth,
-            containingBlockHeight);
-}
-
 void layoutAbsoluteChild(
     const yoga::Node* const containingNode,
     const yoga::Node* const node,