Scale input decorator label width (#59010)

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

@@ -17,6 +17,7 @@ import 'theme_data.dart';
 
 const Duration _kTransitionDuration = Duration(milliseconds: 200);
 const Curve _kTransitionCurve = Curves.fastOutSlowIn;
+const double _kFinalLabelScale = 0.75;
 
 // Defines the gap in the InputDecorator's outline border where the
 // floating label will appear.
@@ -981,9 +982,11 @@ class _RenderDecoration extends RenderBox {
       + _boxSize(suffix).width
       + _boxSize(suffixIcon).width
       + contentPadding.right));
+    // Increase the available width for the label when it is scaled down.
+    final double invertedLabelScale = lerpDouble(1.00, 1 / _kFinalLabelScale, decoration.floatingLabelProgress);
     boxToBaseline[label] = _layoutLineBox(
       label,
-      boxConstraints.copyWith(maxWidth: inputWidth),
+      boxConstraints.copyWith(maxWidth: inputWidth * invertedLabelScale),
     );
     boxToBaseline[hint] = _layoutLineBox(
       hint,
@@ -1452,11 +1455,10 @@ class _RenderDecoration extends RenderBox {
       final bool isOutlineBorder = decoration.border != null && decoration.border.isOutline;
       // Temporary opt-in fix for https://github.com/flutter/flutter/issues/54028
       // Center the scaled label relative to the border.
-      const double finalLabelScale = 0.75;
       final double floatingY = decoration.fixTextFieldOutlineLabel
-        ? isOutlineBorder ? (-labelHeight * finalLabelScale) / 2.0 + borderWeight / 2.0 : contentPadding.top
+        ? isOutlineBorder ? (-labelHeight * _kFinalLabelScale) / 2.0 + borderWeight / 2.0 : contentPadding.top
         : isOutlineBorder ? -labelHeight * 0.25 : contentPadding.top;
-      final double scale = lerpDouble(1.0, finalLabelScale, t);
+      final double scale = lerpDouble(1.0, _kFinalLabelScale, t);
       double dx;
       switch (textDirection) {
         case TextDirection.rtl:

packages/flutter/test/material/input_decorator_test.dart

@@ -4069,6 +4069,58 @@ void main() {
     expect(tester.getTopLeft(find.text('label')).dy, 20.0);
   });
 
+  testWidgets('InputDecorator floating label width scales when focused', (WidgetTester tester) async {
+    final String longStringA = String.fromCharCodes(List<int>.generate(200, (_) => 65));
+    final String longStringB = String.fromCharCodes(List<int>.generate(200, (_) => 66));
+
+    await tester.pumpWidget(
+      Center(
+        child: Container(
+          width: 100,
+          height: 100,
+          child: buildInputDecorator(
+            // isFocused: false (default)
+            isEmpty: true,
+            decoration: InputDecoration(
+              labelText: longStringA,
+            ),
+          ),
+        ),
+      ),
+    );
+
+    await tester.pumpAndSettle();
+
+    expect(
+      find.text(longStringA),
+      paints..clipRect(rect: const Rect.fromLTWH(0, 0, 100.0, 16.0)),
+    );
+
+    await tester.pumpWidget(
+      Center(
+        child: Container(
+          width: 100,
+          height: 100,
+          child: buildInputDecorator(
+            isFocused: true,
+            isEmpty: true,
+            decoration: InputDecoration(
+              labelText: longStringB,
+            ),
+          ),
+        ),
+      ),
+    );
+
+    await tester.pumpAndSettle();
+
+    expect(
+      find.text(longStringB),
+      // 133.3 is approximately 100 / 0.75 (_kFinalLabelScale)
+      paints..clipRect(rect: const Rect.fromLTWH(0, 0, 133.0, 16.0)),
+    );
+  }, skip: isBrowser);  // TODO(yjbanov): https://github.com/flutter/flutter/issues/44020
+
   testWidgets('textAlignVertical can be updated', (WidgetTester tester) async {
     // Regression test for https://github.com/flutter/flutter/issues/56933
     const String hintText = 'hint';