Keep iOS text line breaks stable while LayoutAnimation resizes a paragraph

LayoutAnimation interpolates a paragraph's frame on every frame, and
RCTParagraphComponentView laid out its text in that intermediate size. With
an unlimited number of lines the text container clips its last line instead
of wrapping it, so the last visible line showed words from the next line,
cut off at the edge, and they jumped down once the next line was revealed.

ParagraphState (outside of Android) now stores the size of the content frame
the paragraph was laid out with, and the view lays out text in that size and
clips it to its current bounds, like the old architecture did. LayoutAnimation
keeps the final state while it interpolates the frame, so this needs no
animation-specific code.

Fixes #59161

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: tomekzaw

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

@@ -188,6 +188,13 @@ - (void)_updateTextViewFrame
   CGRect textViewFrame = self.bounds;
   CGRect drawingFrame = RCTCGRectFromRect(_layoutMetrics.getContentFrame());
 
+  // Lay out the text in the size the paragraph has in the shadow tree rather than in the current frame, which
+  // LayoutAnimation interpolates. Otherwise, line breaks change on every frame and words jump between lines.
+  // The text view clips the text to the current bounds.
+  if (_textView.state) {
+    drawingFrame.size = RCTCGSizeFromSize(_textView.state->getData().layoutSize);
+  }
+
   if (ReactNativeFeatureFlags::enableIOSCompressedTextFrameAdjustment() && _textView.state &&
       drawingFrame.size.height > 0) {
     const auto &stateData = _textView.state->getData();

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

@@ -178,6 +178,7 @@ void ParagraphShadowNode::updateStateIfNeeded(const Content& content) {
   auto& state = getStateData();
 
   react_native_assert(textLayoutManager_);
+#ifdef ANDROID
   if (state.attributedString == content.attributedString) {
     return;
   }
@@ -187,6 +188,20 @@ void ParagraphShadowNode::updateStateIfNeeded(const Content& content) {
           content.attributedString,
           content.paragraphAttributes,
           textLayoutManager_});
+#else
+  auto layoutSize = getLayoutMetrics().getContentFrame().size;
+  if (state.attributedString == content.attributedString &&
+      state.layoutSize == layoutSize) {
+    return;
+  }
+
+  setStateData(
+      ParagraphState{
+          content.attributedString,
+          content.paragraphAttributes,
+          textLayoutManager_,
+          layoutSize});
+#endif
 }
 
 MeasuredPreparedTextLayout* ParagraphShadowNode::findUsableLayout() {

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

@@ -110,8 +110,8 @@ class ParagraphShadowNode
   void updateStateIfNeeded(const Content &content, const MeasuredPreparedTextLayout &layout);
 
   /*
-   * Creates a `State` object (with `AttributedText` and
-   * `TextLayoutManager`) if needed.
+   * Creates a `State` object (with `AttributedText`, `TextLayoutManager` and,
+   * outside of Android, the size of the content frame) if needed.
    */
   void updateStateIfNeeded(const Content &content);
 

packages/react-native/ReactCommon/react/renderer/components/text/platform/cxx/react/renderer/components/text/ParagraphState.h

@@ -10,6 +10,7 @@
 #include <react/cxxstableapi/FrameworksGuard.h>
 
 #include <React/Debug.h>
+#include <React/Graphics.h>
 #include <react/renderer/attributedstring/AttributedString.h>
 #include <react/renderer/attributedstring/ParagraphAttributes.h>
 #include <react/renderer/textlayoutmanager/TextLayoutManager.h>
@@ -39,6 +40,14 @@ class ParagraphState final {
    * `AttributedString`.
    */
   std::weak_ptr<const TextLayoutManager> layoutManager;
+
+  /*
+   * Size of the content frame that the paragraph was laid out with.
+   * The mounted view can temporarily have a different size (e.g. while
+   * LayoutAnimation interpolates its frame), so it lays out text in this size
+   * to keep line breaks stable.
+   */
+  Size layoutSize;
 };
 
 } // namespace facebook::react