Fix floating behavior of long label and outline input border (flutter#68727)

Corrects the space available to the label in an outlined text field

Author: YeungKC

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

@@ -976,11 +976,15 @@ class _RenderDecoration extends RenderBox {
       + contentPadding.right));
     // Increase the available width for the label when it is scaled down.
     final double invertedLabelScale = lerpDouble(1.00, 1 / _kFinalLabelScale, decoration.floatingLabelProgress)!;
+    double suffixIconWidth = _boxSize(suffixIcon).width;
+    if (decoration.border!.isOutline) {
+      suffixIconWidth = lerpDouble(suffixIconWidth, 0.0, decoration.floatingLabelProgress)!;
+    }
     final double labelWidth = math.max(0.0, constraints.maxWidth - (
       _boxSize(icon).width
       + contentPadding.left
       + _boxSize(prefixIcon).width
-      + _boxSize(suffixIcon).width
+      + suffixIconWidth
       + contentPadding.right));
     boxToBaseline[label] = _layoutLineBox(
       label,

packages/flutter/test/material/input_decorator_test.dart

@@ -4253,6 +4253,58 @@ void main() {
     expect(getOpacity(tester, prefixText), 1.0);
   });
 
+  // Related issue: https://github.com/flutter/flutter/issues/64427
+  testWidgets('OutlineInputBorder and InputDecorator long labels and in Floating, the width should ignore the icon width', (WidgetTester tester) async {
+    const String labelText = 'Flutter is Google’s UI toolkit for building beautiful, natively compiled applications for mobile, web, and desktop from a single codebase.';
+
+    Widget getLabeledInputDecorator(FloatingLabelBehavior floatingLabelBehavior) => MaterialApp(
+        home: Material(
+          child: Container(
+            width: 300,
+            child: TextField(
+              decoration: InputDecoration(
+                border: const OutlineInputBorder(
+                  borderSide: BorderSide(color: Colors.greenAccent, width: 1.0),
+                ),
+                suffixIcon: const Icon(Icons.arrow_drop_down),
+                floatingLabelBehavior: floatingLabelBehavior,
+                labelText: labelText,
+              ),
+            ),
+          ),
+        ),
+      );
+
+    await tester.pumpWidget(getLabeledInputDecorator(FloatingLabelBehavior.never));
+
+    final double labelWidth = getLabelRect(tester).width;
+
+    await tester.pumpWidget(getLabeledInputDecorator(FloatingLabelBehavior.always));
+    await tester.pumpAndSettle();
+
+    final double floatedLabelWidth = getLabelRect(tester).width;
+
+    expect(floatedLabelWidth > labelWidth, isTrue);
+
+    final Widget target = getLabeledInputDecorator(FloatingLabelBehavior.auto);
+    await tester.pumpWidget(target);
+    await tester.pumpAndSettle();
+
+    expect(getLabelRect(tester).width, labelWidth);
+
+    // Click for Focus.
+    await tester.tap(find.byType(TextField));
+    // Default animation duration is 200 millisecond.
+    await tester.pumpFrames(target, const Duration(milliseconds: 100));
+
+    expect(getLabelRect(tester).width > labelWidth, isTrue);
+    expect(getLabelRect(tester).width < floatedLabelWidth, isTrue);
+
+    await tester.pumpAndSettle();
+
+    expect(getLabelRect(tester).width, floatedLabelWidth);
+  });
+
   testWidgets('given enough space, constrained and unconstrained heights result in the same size widget', (WidgetTester tester) async {
     // Regression test for https://github.com/flutter/flutter/issues/65572
     final UniqueKey keyUnconstrained = UniqueKey();