Fix InputDecorator label padding (flutter#173344)

## Description

This PR fixes the label padding for an InputDecorator with prefixIcon
and/or suffixIcon.

# Before

The label was shorter than the available space because
`InputDecoration.contentPadding.horizontal` was applied even when
`prefixIcon` and `suffixIcon` were defined. The icons width replaces the
corresponding `contentPadding`, so both should not be used at the same
time to compute the available space.

<img width="269" height="61" alt="image"
src="https://github.com/user-attachments/assets/e1433ffb-8f17-46fe-9a65-6b9a504ef043"
/>

# After

The label takes all the available space.

<img width="269" height="61" alt="image"
src="https://github.com/user-attachments/assets/12e8d087-c75b-48c9-8df4-1b11207c0e73"
/>


## Related Issue

Fixes [Label padding is wrong for InputDecorator with prefixIcon and/or
suffixIcon ](flutter#173341)

## Tests

Adds 1 test.

Author: bleroux

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

@@ -1404,16 +1404,16 @@ class _RenderDropdownMenuBody extends RenderBox
         child = childParentData.nextSibling;
         continue;
       }
-      final double maxIntrinsicWidth = child.getMinIntrinsicWidth(height);
+      final double minIntrinsicWidth = child.getMinIntrinsicWidth(height);
       // Add the width of leading icon.
       if (child == lastChild) {
-        width += maxIntrinsicWidth;
+        width += minIntrinsicWidth;
       }
       // Add the width of trailing icon.
       if (child == childBefore(lastChild!)) {
-        width += maxIntrinsicWidth;
+        width += minIntrinsicWidth;
       }
-      width = math.max(width, maxIntrinsicWidth);
+      width = math.max(width, minIntrinsicWidth);
       final _DropdownMenuBodyParentData childParentData =
           child.parentData! as _DropdownMenuBodyParentData;
       child = childParentData.nextSibling;

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

@@ -1012,16 +1012,15 @@ class _RenderDecoration extends RenderBox
     final double topHeight;
     if (label != null) {
       final double suffixIconSpace = decoration.border.isOutline
-          ? lerpDouble(suffixIconSize.width, 0.0, decoration.floatingLabelProgress)!
+          ? lerpDouble(suffixIconSize.width, contentPadding.end, decoration.floatingLabelProgress)!
           : suffixIconSize.width;
       final double labelWidth = math.max(
         0.0,
         constraints.maxWidth -
             (decoration.inputGap * 2 +
                 iconWidth +
-                contentPadding.horizontal +
-                prefixIconSize.width +
-                suffixIconSpace),
+                (prefixIcon == null ? contentPadding.start : prefixIconSize.width) +
+                (suffixIcon == null ? contentPadding.end : suffixIconSpace)),
       );
 
       // Increase the available width for the label when it is scaled down.

packages/flutter/test/material/input_decorator_test.dart

@@ -2951,6 +2951,71 @@ void main() {
       // TODO(bleroux): fix input decorator to not rely on forcing the label text line height to 1.0.
     });
 
+    testWidgets('When the label appears within the input its padding is correct', (
+      WidgetTester tester,
+    ) async {
+      // Define a label larger than the available decorator, the label will fill
+      // all the available space (decorator width minus padding and affixes).
+      const Widget largeLabel = SizedBox(key: customLabelKey, width: 1000, height: 16);
+      await tester.pumpWidget(
+        buildInputDecorator(
+          isEmpty: true,
+          decoration: const InputDecoration(filled: true, label: largeLabel),
+        ),
+      );
+
+      // For filled and/or outlined decoration, the horizontal padding is 16.
+      const double horizontalPadding = 16.0;
+      expect(getCustomLabelRect(tester).left, horizontalPadding);
+      expect(getCustomLabelRect(tester).right, 800 - horizontalPadding);
+
+      // Outlined decorator.
+      await tester.pumpWidget(
+        buildInputDecorator(
+          isEmpty: true,
+          decoration: const InputDecoration(border: OutlineInputBorder(), label: largeLabel),
+        ),
+      );
+
+      expect(getCustomLabelRect(tester).left, horizontalPadding);
+      expect(getCustomLabelRect(tester).right, 800 - horizontalPadding);
+
+      // Rebuild with affixes.
+      await tester.pumpWidget(
+        buildInputDecorator(
+          isEmpty: true,
+          decoration: const InputDecoration(
+            filled: true,
+            label: largeLabel,
+            suffixIcon: Icon(Icons.align_horizontal_left_sharp),
+            prefixIcon: Icon(Icons.align_horizontal_right_sharp),
+          ),
+        ),
+      );
+
+      // When suffixIcon and/or prefixIcon are set, the corresponding horizontal
+      // padding is 52 (48 for the icon + 4 input gap based on M3 spec).
+      const double affixesHorizontalPadding = 52.0;
+      expect(getCustomLabelRect(tester).left, affixesHorizontalPadding);
+      expect(getCustomLabelRect(tester).right, 800 - affixesHorizontalPadding);
+
+      // Outlined decorator.
+      await tester.pumpWidget(
+        buildInputDecorator(
+          isEmpty: true,
+          decoration: const InputDecoration(
+            border: OutlineInputBorder(),
+            label: largeLabel,
+            suffixIcon: Icon(Icons.align_horizontal_left_sharp),
+            prefixIcon: Icon(Icons.align_horizontal_right_sharp),
+          ),
+        ),
+      );
+
+      expect(getCustomLabelRect(tester).left, affixesHorizontalPadding);
+      expect(getCustomLabelRect(tester).right, 800 - affixesHorizontalPadding);
+    });
+
     testWidgets(
       'The label appears above the input when there is no content and floatingLabelBehavior is always',
       (WidgetTester tester) async {
@@ -8770,15 +8835,19 @@ void main() {
           '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: SizedBox(
-            width: 300,
-            child: TextField(
-              decoration: InputDecoration(
-                border: const OutlineInputBorder(borderSide: BorderSide(color: Colors.greenAccent)),
-                suffixIcon: const Icon(Icons.arrow_drop_down),
-                floatingLabelBehavior: floatingLabelBehavior,
-                labelText: labelText,
+        home: MaterialApp(
+          home: Scaffold(
+            body: SizedBox(
+              width: 300,
+              child: TextField(
+                decoration: InputDecoration(
+                  border: const OutlineInputBorder(
+                    borderSide: BorderSide(color: Colors.greenAccent),
+                  ),
+                  suffixIcon: const Icon(Icons.arrow_drop_down),
+                  floatingLabelBehavior: floatingLabelBehavior,
+                  labelText: labelText,
+                ),
               ),
             ),
           ),