Round node dimensions in double so far-off nodes keep their full size

tests/YGRoundingLargeOffsetTest.cpp

@@ -0,0 +1,54 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#include <gtest/gtest.h>
+#include <yoga/Yoga.h>
+
+static YGSize _measureTwoLines(
+    YGNodeConstRef /*node*/,
+    float /*width*/,
+    YGMeasureMode /*widthMode*/,
+    float /*height*/,
+    YGMeasureMode /*heightMode*/) {
+  return YGSize{292.0f, 44.0f};
+}
+
+// A measured (text) node far from the origin on a 3x screen, with its top on
+// one side of a power of two and its bottom on the other. Each edge is rounded
+// to the pixel grid and the height is their difference; in float the two
+// edges have different precision and the height came out one step short (for
+// example 43.9998779 instead of 44 for a node spanning 2048), so a text
+// renderer given that box dropped the last line.
+TEST(YogaTest, rounding_measured_height_is_exact_across_float_steps) {
+  for (const float spacer :
+       {1012.0f + 1.0f / 3.0f,
+        1012.0f + 2.0f / 3.0f,
+        2004.0f + 1.0f / 3.0f,
+        2028.0f + 2.0f / 3.0f,
+        4052.0f + 2.0f / 3.0f}) {
+    YGConfigRef config = YGConfigNew();
+    YGConfigSetPointScaleFactor(config, 3.0f);
+
+    YGNodeRef root = YGNodeNewWithConfig(config);
+    YGNodeStyleSetWidth(root, 300);
+
+    YGNodeRef spacerNode = YGNodeNewWithConfig(config);
+    YGNodeStyleSetHeight(spacerNode, spacer);
+    YGNodeInsertChild(root, spacerNode, 0);
+
+    YGNodeRef text = YGNodeNewWithConfig(config);
+    YGNodeSetMeasureFunc(text, _measureTwoLines);
+    YGNodeInsertChild(root, text, 1);
+
+    YGNodeCalculateLayout(root, YGUndefined, YGUndefined, YGDirectionLTR);
+
+    ASSERT_EQ(44.0f, YGNodeLayoutGetHeight(text)) << "spacer " << spacer;
+
+    YGNodeFreeRecursive(root);
+    YGConfigFree(config);
+  }
+}

yoga/algorithm/PixelGrid.cpp

@@ -12,7 +12,14 @@
 
 namespace facebook::yoga {
 
-float roundValueToPixelGrid(
+namespace {
+
+// Rounds in double precision. A dimension is the difference of two rounded
+// edges, and far from the origin a float has fewer bits than a pixel: taking
+// that difference in float can leave it a step short of the real size (for
+// example 43.9998 instead of 44 for a node spanning y = 2048 on a 3x screen),
+// which makes a text renderer drop the last line. Subtract first, then narrow.
+double roundValueToPixelGridDouble(
     const double value,
     const double pointScaleFactor,
     const bool forceCeil,
@@ -60,7 +67,18 @@ float roundValueToPixelGrid(
   }
   return (std::isnan(scaledValue) || std::isnan(pointScaleFactor))
       ? YGUndefined
-      : (float)(scaledValue / pointScaleFactor);
+      : scaledValue / pointScaleFactor;
+}
+
+} // namespace
+
+float roundValueToPixelGrid(
+    const double value,
+    const double pointScaleFactor,
+    const bool forceCeil,
+    const bool forceFloor) {
+  return static_cast<float>(roundValueToPixelGridDouble(
+      value, pointScaleFactor, forceCeil, forceFloor));
 }
 
 void roundLayoutResultsToPixelGrid(
@@ -109,23 +127,25 @@ void roundLayoutResultsToPixelGrid(
 
     node->getLayout().setDimension(
         Dimension::Width,
-        roundValueToPixelGrid(
-            absoluteNodeRight,
-            pointScaleFactor,
-            (textRounding && hasFractionalWidth),
-            (textRounding && !hasFractionalWidth)) -
-            roundValueToPixelGrid(
-                absoluteNodeLeft, pointScaleFactor, false, textRounding));
+        static_cast<float>(
+            roundValueToPixelGridDouble(
+                absoluteNodeRight,
+                pointScaleFactor,
+                (textRounding && hasFractionalWidth),
+                (textRounding && !hasFractionalWidth)) -
+            roundValueToPixelGridDouble(
+                absoluteNodeLeft, pointScaleFactor, false, textRounding)));
 
     node->getLayout().setDimension(
         Dimension::Height,
-        roundValueToPixelGrid(
-            absoluteNodeBottom,
-            pointScaleFactor,
-            (textRounding && hasFractionalHeight),
-            (textRounding && !hasFractionalHeight)) -
-            roundValueToPixelGrid(
-                absoluteNodeTop, pointScaleFactor, false, textRounding));
+        static_cast<float>(
+            roundValueToPixelGridDouble(
+                absoluteNodeBottom,
+                pointScaleFactor,
+                (textRounding && hasFractionalHeight),
+                (textRounding && !hasFractionalHeight)) -
+            roundValueToPixelGridDouble(
+                absoluteNodeTop, pointScaleFactor, false, textRounding)));
   }
 
   for (yoga::Node* child : node->getChildren()) {