Text: keep the last line when Yoga's pixel-grid rounding leaves the box a float step short

packages/react-native/React/Fabric/Mounting/ComponentViews/Text/RCTParagraphComponentView.mm

@@ -20,11 +20,34 @@
 #import <react/renderer/textlayoutmanager/TextLayoutManager.h>
 #import <react/utils/ManagedObjectWrapper.h>
 
+#import <React/RCTUtils.h>
 #import "RCTConversions.h"
 #import "RCTFabricComponentsPlugins.h"
 
 using namespace facebook::react;
 
+/*
+ * The content frame with its height snapped back onto the pixel grid. Yoga
+ * rounds the frame to the grid but subtracts the two rounded edges as floats,
+ * so far from the origin the height can come out a float step under the grid
+ * value the paragraph was measured at: 43.9998 instead of 44 for two lines of
+ * lineHeight 22 at y = 2048 on a 3x screen. TextKit treats a container that
+ * short as too small for the last line and lays it out clipped. Anything else
+ * is rounded up, so the container is never made smaller.
+ * See https://github.com/facebook/react-native/issues/58970.
+ */
+static CGRect RCTParagraphTextFrame(const LayoutMetrics &layoutMetrics)
+{
+  CGRect frame = RCTCGRectFromRect(layoutMetrics.getContentFrame());
+  CGFloat scale = RCTScreenScale();
+  if (scale > 0 && !isnan(frame.size.height)) {
+    CGFloat scaled = frame.size.height * scale;
+    CGFloat nearest = round(scaled);
+    frame.size.height = (fabs(scaled - nearest) < 0.01 ? nearest : ceil(scaled)) / scale;
+  }
+  return frame;
+}
+
 @interface RCTTextLayoutManager (RCTParagraphComponentViewPrivate)
 
 - (CGRect)drawingFrameForAttributedString:(facebook::react::AttributedString)attributedString
@@ -178,7 +201,7 @@ - (void)prepareForRecycle
 - (void)_updateTextViewFrame
 {
   CGRect textViewFrame = self.bounds;
-  CGRect drawingFrame = RCTCGRectFromRect(_layoutMetrics.getContentFrame());
+  CGRect drawingFrame = RCTParagraphTextFrame(_layoutMetrics);
 
   if (ReactNativeFeatureFlags::enableIOSCompressedTextFrameAdjustment() && _textView.state &&
       drawingFrame.size.height > 0) {
@@ -474,4 +497,4 @@ - (void)drawRect:(CGRect)rect
                               }];
 }
 
-@end
+@end

packages/react-native/ReactCommon/react/renderer/components/text/ParagraphShadowNode.cpp

@@ -33,6 +33,31 @@
           (layoutConstraints).maximumSize.height)
 
 namespace facebook::react {
+
+namespace {
+
+/*
+ * Yoga rounds a node's frame to the pixel grid, but it subtracts the two
+ * rounded edges as floats, so far from the origin the height can come out a
+ * float step under the grid value the paragraph was measured at: 43.9998
+ * instead of 44 for two lines of lineHeight 22 at y = 2048 on a 3x screen.
+ * Laid out in a container that short, the last line does not fit and is
+ * drawn clipped. Snap such a height back onto the grid; anything else is
+ * rounded up, so a container is never made smaller.
+ * See https://github.com/facebook/react-native/issues/58970.
+ */
+Float snapHeightToPixelGrid(Float height, Float pointScaleFactor) {
+  if (pointScaleFactor <= 0 || std::isnan(height)) {
+    return height;
+  }
+  const auto scaled = height * pointScaleFactor;
+  const auto nearest = std::round(scaled);
+  const auto snapped =
+      std::abs(scaled - nearest) < 0.01f ? nearest : std::ceil(scaled);
+  return snapped / pointScaleFactor;
+}
+
+} // namespace
 using Content = ParagraphShadowNode::Content;
 
 // NOLINTNEXTLINE(facebook-hte-CArray, modernize-avoid-c-arrays)
@@ -305,6 +330,10 @@ void ParagraphShadowNode::layout(LayoutContext layoutContext) {
   auto size = ReactNativeFeatureFlags::enablePreparedTextLayout()
       ? rawContentSize()
       : layoutMetrics.getContentFrame().size;
+  if (!ReactNativeFeatureFlags::enablePreparedTextLayout()) {
+    size.height =
+        snapHeightToPixelGrid(size.height, layoutContext.pointScaleFactor);
+  }
 
   LayoutConstraints layoutConstraints{
       .minimumSize = size,
@@ -445,4 +474,4 @@ void ParagraphShadowNode::layout(LayoutContext layoutContext) {
   }
 }
 
-} // namespace facebook::react
+} // namespace facebook::react