Improve `TextPainter.layout` caching (#118128)

Improves `TextPainter.layout` caching when only the input constraints change: 
- removes the double layout calls in `TextPainter._layoutParagraph`: now double layout is only needed when `TextAlign` is not left, and the input `maxWidth == double.infinity`.  
- skip calls to `ui.Paragraph.layout` when it's guaranteed that there's no soft line breaks before/after the layout call.

This doesn't introduce new APIs but may slightly shift text rendered on screen.
This reduces the number of `layout` calls but since shaping results are already cached so it only skips the relatively cheap line-breaking process when possible.

528 scuba failures but all of them seem reasonable.

packages/flutter/lib/src/material/range_slider.dart

@@ -1129,6 +1129,7 @@ class _RenderRangeSlider extends RenderBox with RelayoutWhenSystemFontsChangeMix
   }
 
   void _updateLabelPainter(Thumb thumb) {
+    final RangeLabels? labels = this.labels;
     if (labels == null) {
       return;
     }
@@ -1137,14 +1138,13 @@ class _RenderRangeSlider extends RenderBox with RelayoutWhenSystemFontsChangeMix
     final TextPainter labelPainter;
     switch (thumb) {
       case Thumb.start:
-        text = labels!.start;
+        text = labels.start;
         labelPainter = _startLabelPainter;
       case Thumb.end:
-        text = labels!.end;
+        text = labels.end;
         labelPainter = _endLabelPainter;
     }
 
-    if (labels != null) {
     labelPainter
       ..text = TextSpan(
         style: _sliderTheme.valueIndicatorTextStyle,
@@ -1153,9 +1153,6 @@ class _RenderRangeSlider extends RenderBox with RelayoutWhenSystemFontsChangeMix
       ..textDirection = textDirection
       ..textScaleFactor = textScaleFactor
       ..layout();
-    } else {
-      labelPainter.text = null;
-    }
     // Changing the textDirection can result in the layout changing, because the
     // bidi algorithm might line up the glyphs differently which can result in
     // different ligatures, different shapes, etc. So we always markNeedsLayout.

packages/flutter/test/material/input_decorator_test.dart

@@ -5692,8 +5692,15 @@ void main() {
 
     expect(
       find.text(longStringB),
+      paints..something((Symbol methodName, List<dynamic> arguments) {
+        if (methodName != #clipRect) {
+          return false;
+        }
+        final Rect clipRect = arguments[0] as Rect;
         // 133.3 is approximately 100 / 0.75 (_kFinalLabelScale)
-      paints..clipRect(rect: const Rect.fromLTWH(0, 0, 133.0, 16.0)),
+        expect(clipRect, rectMoreOrLessEquals(const Rect.fromLTWH(0, 0, 133.0, 16.0)));
+        return true;
+      }),
     );
   }, skip: isBrowser);  // TODO(yjbanov): https://github.com/flutter/flutter/issues/44020
 

packages/flutter/test/painting/text_painter_test.dart

@@ -382,9 +382,9 @@ void main() {
 
   test('TextPainter requires textDirection', () {
     final TextPainter painter1 = TextPainter(text: const TextSpan(text: ''));
-    expect(() { painter1.layout(); }, throwsAssertionError);
+    expect(painter1.layout, throwsStateError);
     final TextPainter painter2 = TextPainter(text: const TextSpan(text: ''), textDirection: TextDirection.rtl);
-    expect(() { painter2.layout(); }, isNot(throwsException));
+    expect(painter2.layout, isNot(throwsStateError));
   });
 
   test('TextPainter size test', () {
@@ -1457,8 +1457,67 @@ void main() {
 
     painter.dispose();
   });
+
+  test('TextPainter infinite width - centered', () {
+    final TextPainter painter = TextPainter()
+      ..textAlign = TextAlign.center
+      ..textDirection = TextDirection.ltr;
+    painter.text = const TextSpan(text: 'A', style: TextStyle(fontSize: 10));
+    MockCanvasWithDrawParagraph mockCanvas = MockCanvasWithDrawParagraph();
+
+    painter.layout(minWidth: double.infinity);
+    expect(painter.width, double.infinity);
+    expect(() => painter.paint(mockCanvas = MockCanvasWithDrawParagraph(), Offset.zero), returnsNormally);
+    expect(mockCanvas.centerX, isNull);
+
+    painter.layout();
+    expect(painter.width, 10);
+    expect(() => painter.paint(mockCanvas = MockCanvasWithDrawParagraph(), Offset.zero), returnsNormally);
+    expect(mockCanvas.centerX, 5);
+
+    painter.layout(minWidth: 100);
+    expect(painter.width, 100);
+    expect(() => painter.paint(mockCanvas = MockCanvasWithDrawParagraph(), Offset.zero), returnsNormally);
+    expect(mockCanvas.centerX, 50);
+
+    painter.dispose();
+  });
+
+  test('TextPainter infinite width - LTR justified', () {
+    final TextPainter painter = TextPainter()
+      ..textAlign = TextAlign.justify
+      ..textDirection = TextDirection.ltr;
+    painter.text = const TextSpan(text: 'A', style: TextStyle(fontSize: 10));
+    MockCanvasWithDrawParagraph mockCanvas = MockCanvasWithDrawParagraph();
+
+    painter.layout(minWidth: double.infinity);
+    expect(painter.width, double.infinity);
+    expect(() => painter.paint(mockCanvas = MockCanvasWithDrawParagraph(), Offset.zero), returnsNormally);
+    expect(mockCanvas.offsetX, 0);
+
+    painter.layout();
+    expect(painter.width, 10);
+    expect(() => painter.paint(mockCanvas = MockCanvasWithDrawParagraph(), Offset.zero), returnsNormally);
+    expect(mockCanvas.offsetX, 0);
+
+    painter.layout(minWidth: 100);
+    expect(painter.width, 100);
+    expect(() => painter.paint(mockCanvas = MockCanvasWithDrawParagraph(), Offset.zero), returnsNormally);
+    expect(mockCanvas.offsetX, 0);
+
+    painter.dispose();
+  });
 }
 
 class MockCanvas extends Fake implements Canvas {
+}
 
+class MockCanvasWithDrawParagraph extends Fake implements Canvas {
+  double? centerX;
+  double? offsetX;
+  @override
+  void drawParagraph(ui.Paragraph paragraph, Offset offset) {
+    offsetX = offset.dx;
+    centerX = offset.dx + paragraph.width / 2;
+  }
 }

packages/flutter/test/widgets/text_test.dart

@@ -1225,9 +1225,11 @@ void main() {
               width: 400,
               child: Center(
                 child: RichText(
+                  // 400 is not wide enough for this string. The part after the
+                  // whitespace is going to be broken into a 2nd line.
                   text: const TextSpan(text: 'fwefwefwewfefewfwe fwfwfwefweabcdefghijklmnopqrstuvwxyz'),
                   textWidthBasis: TextWidthBasis.longestLine,
-                  textDirection: TextDirection.ltr,
+                  textDirection: TextDirection.rtl,
                 ),
               ),
             ),
@@ -1239,11 +1241,12 @@ void main() {
           return false;
         }
         final ui.Paragraph paragraph = arguments[0] as ui.Paragraph;
-        if (paragraph.longestLine > paragraph.width) {
-          throw 'paragraph width (${paragraph.width}) greater than its longest line (${paragraph.longestLine}).';
+        final Offset offset = arguments[1] as Offset;
+        final List<ui.LineMetrics> lines = paragraph.computeLineMetrics();
+        for (final ui.LineMetrics line in lines) {
+          if (line.left + offset.dx + line.width >= 400) {
+            throw 'line $line is greater than the max width constraints';
           }
-        if (paragraph.width >= 400) {
-          throw 'paragraph.width (${paragraph.width}) >= 400';
         }
         return true;
       }));