Fix text painter longest line resizing logic for `TextWidthBasis.longestLine` (#143024)

Fixes https://github.com/flutter/flutter/issues/142309.

dev/integration_tests/flutter_gallery/lib/demo/shrine/supplemental/product_card.dart

@@ -35,7 +35,7 @@ class ProductCard extends StatelessWidget {
 
     // The fontSize to use for computing the heuristic UI scaling factor.
     const double defaultFontSize = 14.0;
-    final double containerScaingFactor = MediaQuery.textScalerOf(context).scale(defaultFontSize) / defaultFontSize;
+    final double containerScalingFactor = MediaQuery.textScalerOf(context).scale(defaultFontSize) / defaultFontSize;
 
     return ScopedModelDescendant<AppStateModel>(
       builder: (BuildContext context, Widget? child, AppStateModel model) {
@@ -56,7 +56,7 @@ class ProductCard extends StatelessWidget {
                 child: imageWidget,
               ),
               SizedBox(
-                height: kTextBoxHeight * containerScaingFactor,
+                height: kTextBoxHeight * containerScalingFactor,
                 width: 121.0,
                 child: Column(
                   mainAxisAlignment: MainAxisAlignment.end,

dev/integration_tests/flutter_gallery/lib/gallery/home.dart

@@ -180,7 +180,7 @@ class _DemoItem extends StatelessWidget {
     final bool isDark = theme.brightness == Brightness.dark;
     // The fontSize to use for computing the heuristic UI scaling factor.
     const double defaultFontSize = 14.0;
-    final double containerScaingFactor = MediaQuery.textScalerOf(context).scale(defaultFontSize) / defaultFontSize;
+    final double containerScalingFactor = MediaQuery.textScalerOf(context).scale(defaultFontSize) / defaultFontSize;
     return RawMaterialButton(
       splashColor: theme.primaryColor.withOpacity(0.12),
       highlightColor: Colors.transparent,
@@ -188,7 +188,7 @@ class _DemoItem extends StatelessWidget {
         _launchDemo(context);
       },
       child: Container(
-        constraints: BoxConstraints(minHeight: _kDemoItemHeight * containerScaingFactor),
+        constraints: BoxConstraints(minHeight: _kDemoItemHeight * containerScalingFactor),
         child: Row(
           children: <Widget>[
             Container(

packages/flutter/lib/src/painting/text_painter.dart

@@ -314,6 +314,13 @@ class _TextLayout {
       TextBaseline.ideographic => _paragraph.ideographicBaseline,
     };
   }
+
+  double _contentWidthFor(double minWidth, double maxWidth, TextWidthBasis widthBasis) {
+    return switch (widthBasis) {
+      TextWidthBasis.longestLine => clampDouble(longestLine, minWidth, maxWidth),
+      TextWidthBasis.parent => clampDouble(maxIntrinsicLineExtent, minWidth, maxWidth),
+    };
+  }
 }
 
 // This class stores the current text layout and the corresponding
@@ -321,14 +328,18 @@ class _TextLayout {
 // depends on the current text layout, which will be invalidated as soon as the
 // text layout is invalidated.
 class _TextPainterLayoutCacheWithOffset {
-  _TextPainterLayoutCacheWithOffset(this.layout, this.textAlignment, double minWidth, double maxWidth, TextWidthBasis widthBasis)
-    : contentWidth = _contentWidthFor(minWidth, maxWidth, widthBasis, layout),
-      assert(textAlignment >= 0.0 && textAlignment <= 1.0);
+  _TextPainterLayoutCacheWithOffset(this.layout, this.textAlignment, this.layoutMaxWidth, this.contentWidth)
+    : assert(textAlignment >= 0.0 && textAlignment <= 1.0),
+      assert(!layoutMaxWidth.isNaN),
+      assert(!contentWidth.isNaN);
 
   final _TextLayout layout;
 
+  // The input width used to lay out the paragraph.
+  final double layoutMaxWidth;
+
   // The content width the text painter should report in TextPainter.width.
-  // This is also used to compute `paintOffset`
+  // This is also used to compute `paintOffset`.
   double contentWidth;
 
   // The effective text alignment in the TextPainter's canvas. The value is
@@ -352,20 +363,14 @@ class _TextPainterLayoutCacheWithOffset {
 
   ui.Paragraph get paragraph => layout._paragraph;
 
-  static double _contentWidthFor(double minWidth, double maxWidth, TextWidthBasis widthBasis, _TextLayout layout) {
-    return switch (widthBasis) {
-      TextWidthBasis.longestLine => clampDouble(layout.longestLine, minWidth, maxWidth),
-      TextWidthBasis.parent => clampDouble(layout.maxIntrinsicLineExtent, minWidth, maxWidth),
-    };
-  }
-
   // Try to resize the contentWidth to fit the new input constraints, by just
   // adjusting the paint offset (so no line-breaking changes needed).
   //
-  // Returns false if the new constraints require re-computing the line breaks,
-  // in which case no side effects will occur.
+  // Returns false if the new constraints require the text layout library to
+  // re-compute the line breaks.
   bool _resizeToFit(double minWidth, double maxWidth, TextWidthBasis widthBasis) {
     assert(layout.maxIntrinsicLineExtent.isFinite);
+    assert(minWidth <= maxWidth);
     // The assumption here is that if a Paragraph's width is already >= its
     // maxIntrinsicWidth, further increasing the input width does not change its
     // layout (but may change the paint offset if it's not left-aligned). This is
@@ -377,21 +382,30 @@ class _TextPainterLayoutCacheWithOffset {
     // of double.infinity, and to make the text visible the paintOffset.dx is
     // bound to be double.negativeInfinity, which invalidates all arithmetic
     // operations.
-    final double newContentWidth = _contentWidthFor(minWidth, maxWidth, widthBasis, layout);
-    if (newContentWidth == contentWidth) {
+
+    if (maxWidth == contentWidth && minWidth == contentWidth) {
+      contentWidth = layout._contentWidthFor(minWidth, maxWidth, widthBasis);
       return true;
     }
-    assert(minWidth <= maxWidth);
-    // Always needsLayout when the current paintOffset and the paragraph width are not finite.
+
+    // Special case:
+    // When the paint offset and the paragraph width are both +∞, it's likely
+    // that the text layout engine skipped layout because there weren't anything
+    // to paint. Always try to re-compute the text layout.
     if (!paintOffset.dx.isFinite && !paragraph.width.isFinite && minWidth.isFinite) {
       assert(paintOffset.dx == double.infinity);
       assert(paragraph.width == double.infinity);
       return false;
     }
+
     final double maxIntrinsicWidth = paragraph.maxIntrinsicWidth;
-    if ((paragraph.width - maxIntrinsicWidth) > -precisionErrorTolerance && (maxWidth - maxIntrinsicWidth) > -precisionErrorTolerance) {
-      // Adjust the paintOffset and contentWidth to the new input constraints.
-      contentWidth = newContentWidth;
+    // Skip line breaking if the input width remains the same, of there will be
+    // no soft breaks.
+    final bool skipLineBreaking = maxWidth == layoutMaxWidth  // Same input max width so relayout is unnecessary.
+      || ((paragraph.width - maxIntrinsicWidth) > -precisionErrorTolerance && (maxWidth - maxIntrinsicWidth) > -precisionErrorTolerance);
+    if (skipLineBreaking) {
+      // Adjust the content width in case the TextWidthBasis changed.
+      contentWidth = layout._contentWidthFor(minWidth, maxWidth, widthBasis);
       return true;
     }
     return false;
@@ -631,10 +645,6 @@ class TextPainter {
   // recreated. The caller may not call `layout` again after text color is
   // updated. See: https://github.com/flutter/flutter/issues/85108
   bool _rebuildParagraphForPaint = true;
-  // `_layoutCache`'s input width. This is only needed because there's no API to
-  // create paint only updates that don't affect the text layout (e.g., changing
-  // the color of the text), on ui.Paragraph or ui.ParagraphBuilder.
-  double _inputWidth = double.nan;
 
   bool get _debugAssertTextLayoutIsValid {
     assert(!debugDisposed);
@@ -1127,7 +1137,7 @@ class TextPainter {
     // infinite paint offset.
     final bool adjustMaxWidth = !maxWidth.isFinite && paintOffsetAlignment != 0;
     final double? adjustedMaxWidth = !adjustMaxWidth ? maxWidth : cachedLayout?.layout.maxIntrinsicLineExtent;
-    _inputWidth = adjustedMaxWidth ?? maxWidth;
+    final double layoutMaxWidth = adjustedMaxWidth ?? maxWidth;
 
     // Only rebuild the paragraph when there're layout changes, even when
     // `_rebuildParagraphForPaint` is true. It's best to not eagerly rebuild
@@ -1137,18 +1147,21 @@ class TextPainter {
     // 2. the user could be measuring the text layout so `paint` will never be
     //    called.
     final ui.Paragraph paragraph = (cachedLayout?.paragraph ?? _createParagraph(text))
-      ..layout(ui.ParagraphConstraints(width: _inputWidth));
-    final _TextPainterLayoutCacheWithOffset newLayoutCache = _TextPainterLayoutCacheWithOffset(
-      _TextLayout._(paragraph), paintOffsetAlignment, minWidth, maxWidth, textWidthBasis,
-    );
+      ..layout(ui.ParagraphConstraints(width: layoutMaxWidth));
+    final _TextLayout layout = _TextLayout._(paragraph);
+    final double contentWidth = layout._contentWidthFor(minWidth, maxWidth, textWidthBasis);
+
+    final _TextPainterLayoutCacheWithOffset newLayoutCache;
     // Call layout again if newLayoutCache had an infinite paint offset.
     // This is not as expensive as it seems, line breaking is relatively cheap
     // as compared to shaping.
     if (adjustedMaxWidth == null && minWidth.isFinite) {
       assert(maxWidth.isInfinite);
-      final double newInputWidth = newLayoutCache.layout.maxIntrinsicLineExtent;
+      final double newInputWidth = layout.maxIntrinsicLineExtent;
       paragraph.layout(ui.ParagraphConstraints(width: newInputWidth));
-      _inputWidth = newInputWidth;
+      newLayoutCache = _TextPainterLayoutCacheWithOffset(layout, paintOffsetAlignment, newInputWidth, contentWidth);
+    } else {
+      newLayoutCache = _TextPainterLayoutCacheWithOffset(layout, paintOffsetAlignment, layoutMaxWidth, contentWidth);
     }
     _layoutCache = newLayoutCache;
   }
@@ -1189,8 +1202,8 @@ class TextPainter {
       // Unfortunately even if we know that there is only paint changes, there's
       // no API to only make those updates so the paragraph has to be recreated
       // and re-laid out.
-      assert(!_inputWidth.isNaN);
-      layoutCache.layout._paragraph = _createParagraph(text!)..layout(ui.ParagraphConstraints(width: _inputWidth));
+      assert(!layoutCache.layoutMaxWidth.isNaN);
+      layoutCache.layout._paragraph = _createParagraph(text!)..layout(ui.ParagraphConstraints(width: layoutCache.layoutMaxWidth));
       assert(paragraph.width == layoutCache.layout._paragraph.width);
       paragraph.dispose();
       assert(debugSize == size);

packages/flutter/test/painting/text_painter_test.dart

@@ -1510,6 +1510,24 @@ void main() {
     painter.dispose();
   });
 
+  test('LongestLine TextPainter properly relayout when maxWidth changes.', () {
+    // Regression test for https://github.com/flutter/flutter/issues/142309.
+    final TextPainter painter = TextPainter()
+      ..textAlign = TextAlign.justify
+      ..textWidthBasis = TextWidthBasis.longestLine
+      ..textDirection = TextDirection.ltr
+      ..text = TextSpan(text: 'A' * 100, style: const TextStyle(fontSize: 10));
+
+    painter.layout(maxWidth: 1000);
+    expect(painter.width, 1000);
+
+    painter.layout(maxWidth: 100);
+    expect(painter.width, 100);
+
+    painter.layout(maxWidth: 1000);
+    expect(painter.width, 1000);
+  });
+
   test('TextPainter line breaking does not round to integers', () {
     const double fontSize = 1.25;
     const String text = '12345';