Always relies on floatingLabelStyle when FloatingLabelBehavior.always (#147374)

## Description

With this PR, when `InputDecorator.floatingLabelBehavior` is set to `FloatingLabelBehavior.always` the label style is always set to `InputDecorator.floatingLabelStyle`, previously `InputDecorator.labelStyle` was used when the field was not focused or was empty.  

## Related Issue

Fixes flutter/flutter#147231

## Tests

Adds 1 test for this particular issue and several missing tests.

Author: bleroux

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

@@ -1828,8 +1828,11 @@ class InputDecorator extends StatefulWidget {
   /// Whether the label needs to get out of the way of the input, either by
   /// floating or disappearing.
   ///
-  /// Will withdraw when not empty, or when focused while enabled.
-  bool get _labelShouldWithdraw => !isEmpty || (isFocused && decoration.enabled);
+  /// Will withdraw when not empty, when focused while enabled, or when
+  /// floating behavior is [FloatingLabelBehavior.always].
+  bool get _labelShouldWithdraw => !isEmpty
+      || (isFocused && decoration.enabled)
+      || decoration.floatingLabelBehavior == FloatingLabelBehavior.always;
 
   @override
   State<InputDecorator> createState() => _InputDecoratorState();
@@ -1872,9 +1875,8 @@ class _InputDecoratorState extends State<InputDecorator> with TickerProviderStat
   void initState() {
     super.initState();
 
-    final bool labelIsInitiallyFloating = widget.decoration.floatingLabelBehavior == FloatingLabelBehavior.always
-        || (widget.decoration.floatingLabelBehavior != FloatingLabelBehavior.never &&
-            widget._labelShouldWithdraw);
+    final bool labelIsInitiallyFloating = widget.decoration.floatingLabelBehavior != FloatingLabelBehavior.never
+        && widget._labelShouldWithdraw;
 
     _floatingLabelController = AnimationController(
       duration: _kTransitionDuration,
@@ -1937,8 +1939,7 @@ class _InputDecoratorState extends State<InputDecorator> with TickerProviderStat
     final bool floatBehaviorChanged = widget.decoration.floatingLabelBehavior != old.decoration.floatingLabelBehavior;
 
     if (widget._labelShouldWithdraw != old._labelShouldWithdraw || floatBehaviorChanged) {
-      if (_floatingLabelEnabled
-          && (widget._labelShouldWithdraw || widget.decoration.floatingLabelBehavior == FloatingLabelBehavior.always)) {
+      if (_floatingLabelEnabled && widget._labelShouldWithdraw) {
         _floatingLabelController.forward();
       } else {
         _floatingLabelController.reverse();
@@ -2028,8 +2029,7 @@ class _InputDecoratorState extends State<InputDecorator> with TickerProviderStat
   // hint would.
   bool get _hasInlineLabel {
     return !widget._labelShouldWithdraw
-        && (decoration.labelText != null || decoration.label != null)
-        && decoration.floatingLabelBehavior != FloatingLabelBehavior.always;
+        && (decoration.labelText != null || decoration.label != null);
   }
 
   // If the label is a floating placeholder, it's always shown.

packages/flutter/test/material/input_decorator_test.dart

@@ -2032,6 +2032,137 @@ void main() {
         });
       });
     });
+
+    testWidgets('floatingLabelStyle overrides default style', (WidgetTester tester) async {
+      const TextStyle floatingLabelStyle = TextStyle(color: Colors.indigo, fontSize: 16.0);
+
+      await tester.pumpWidget(
+        buildInputDecorator(
+          isEmpty: true,
+          isFocused: true, // Label appears floating above input field.
+          decoration: const InputDecoration(
+            labelText: labelText,
+            floatingLabelStyle: floatingLabelStyle,
+          ),
+        ),
+      );
+
+      expect(getLabelStyle(tester).color, floatingLabelStyle.color);
+      expect(getLabelStyle(tester).fontSize, floatingLabelStyle.fontSize);
+    });
+
+    testWidgets('floatingLabelStyle defaults to labelStyle', (WidgetTester tester) async {
+      const TextStyle labelStyle = TextStyle(color: Colors.amber, fontSize: 16.0);
+
+      await tester.pumpWidget(
+        buildInputDecorator(
+          isEmpty: true,
+          isFocused: true, // Label appears floating above input field.
+          decoration: const InputDecoration(
+            labelText: labelText,
+            labelStyle: labelStyle,
+          ),
+        ),
+      );
+
+      expect(getLabelStyle(tester).color, labelStyle.color);
+      expect(getLabelStyle(tester).fontSize, labelStyle.fontSize);
+    });
+
+    testWidgets('floatingLabelStyle takes precedence over labelStyle', (WidgetTester tester) async {
+      const TextStyle labelStyle = TextStyle(color: Colors.amber, fontSize: 16.0);
+      const TextStyle floatingLabelStyle = TextStyle(color: Colors.indigo, fontSize: 16.0);
+
+      await tester.pumpWidget(
+        buildInputDecorator(
+          isEmpty: true,
+          isFocused: true, // Label appears floating above input field.
+          decoration: const InputDecoration(
+            labelText: labelText,
+            labelStyle: labelStyle,
+            floatingLabelStyle: floatingLabelStyle,
+          ),
+        ),
+      );
+
+      expect(getLabelStyle(tester).color, floatingLabelStyle.color);
+      expect(getLabelStyle(tester).fontSize, floatingLabelStyle.fontSize);
+    });
+
+    testWidgets('InputDecorationTheme labelStyle overrides default style', (WidgetTester tester) async {
+      const TextStyle labelStyle = TextStyle(color: Colors.amber, fontSize: 16.0);
+
+      await tester.pumpWidget(
+        buildInputDecorator(
+          isEmpty: true, // Label appears inline, on top of the input field.
+          inputDecorationTheme: const InputDecorationTheme(
+            labelStyle: labelStyle,
+          ),
+          decoration: const InputDecoration(
+            labelText: labelText,
+          ),
+        ),
+      );
+
+      expect(getLabelStyle(tester).color, labelStyle.color);
+    });
+
+    testWidgets('InputDecorationTheme floatingLabelStyle overrides default style', (WidgetTester tester) async {
+      const TextStyle floatingLabelStyle = TextStyle(color: Colors.indigo, fontSize: 16.0);
+
+      await tester.pumpWidget(
+        buildInputDecorator(
+          isEmpty: true,
+          isFocused: true, // Label appears floating above input field.
+          inputDecorationTheme: const InputDecorationTheme(
+            floatingLabelStyle: floatingLabelStyle,
+          ),
+          decoration: const InputDecoration(
+            labelText: labelText,
+          ),
+        ),
+      );
+
+      expect(getLabelStyle(tester).color, floatingLabelStyle.color);
+    });
+
+    testWidgets('floatingLabelStyle is always used when FloatingLabelBehavior.always', (WidgetTester tester) async {
+      // Regression test for https://github.com/flutter/flutter/issues/147231.
+      const TextStyle labelStyle = TextStyle(color: Colors.amber, fontSize: 16.0);
+      const TextStyle floatingLabelStyle = TextStyle(color: Colors.indigo, fontSize: 16.0);
+
+      await tester.pumpWidget(
+        buildInputDecorator(
+          isEmpty: true,
+          decoration: const InputDecoration(
+            labelText: labelText,
+            labelStyle: labelStyle,
+            floatingLabelStyle: floatingLabelStyle,
+            floatingLabelBehavior: FloatingLabelBehavior.always,
+          ),
+        ),
+      );
+
+      expect(getLabelStyle(tester).color, floatingLabelStyle.color);
+      expect(getLabelStyle(tester).fontSize, floatingLabelStyle.fontSize);
+
+      // Focus the input decorator.
+      await tester.pumpWidget(
+        buildInputDecorator(
+          isEmpty: true,
+          isFocused: true,
+          decoration: const InputDecoration(
+            labelText: labelText,
+            labelStyle: labelStyle,
+            floatingLabelStyle: floatingLabelStyle,
+            floatingLabelBehavior: FloatingLabelBehavior.always,
+          ),
+        ),
+      );
+
+      expect(getLabelStyle(tester).color, floatingLabelStyle.color);
+      expect(getLabelStyle(tester).fontSize, floatingLabelStyle.fontSize);
+    });
   });
 
   group('Material3 - InputDecoration labelText layout', () {
@@ -4915,6 +5046,49 @@ void main() {
     expect(getOpacity(tester, prefixText), 1.0);
   });
 
+  testWidgets('Prefix and suffix are not visible when decorator is empty', (WidgetTester tester) async {
+    const String prefixText = 'Prefix';
+    const String suffixText = 'Suffix';
+
+    await tester.pumpWidget(
+      buildInputDecorator(
+        isEmpty: true,
+        decoration: const InputDecoration(
+          filled: true,
+          labelText: labelText,
+          prefixText: prefixText,
+          suffixText: suffixText,
+        ),
+      ),
+    );
+
+    // Prefix and suffix are hidden.
+    expect(getOpacity(tester, prefixText), 0.0);
+    expect(getOpacity(tester, suffixText), 0.0);
+  });
+
+  testWidgets('Prefix and suffix are visible when decorator is empty and floating behavior is FloatingBehavior.always', (WidgetTester tester) async {
+    const String prefixText = 'Prefix';
+    const String suffixText = 'Suffix';
+
+    await tester.pumpWidget(
+      buildInputDecorator(
+        isEmpty: true,
+        decoration: const InputDecoration(
+          filled: true,
+          labelText: labelText,
+          prefixText: prefixText,
+          suffixText: suffixText,
+          floatingLabelBehavior: FloatingLabelBehavior.always,
+        ),
+      ),
+    );
+
+    // Prefix and suffix are visible.
+    expect(getOpacity(tester, prefixText), 1.0);
+    expect(getOpacity(tester, suffixText), 1.0);
+  });
+
   testWidgets('OutlineInputBorder and InputDecorator long labels and in Floating, the width should ignore the icon width', (WidgetTester tester) async {
     // Regression test for https://github.com/flutter/flutter/issues/64427.
     const String labelText = 'Flutter is Google’s UI toolkit for building beautiful, natively compiled applications for mobile, web, and desktop from a single codebase.';