Fix collapsed InputDecorator minimum height (#150770)

## Description

This PR sets a minimal height for collapsed `InputDecoration`. 

Before this PR the minimum height was 0. On desktop, due to visual density reducing the height by 8 pixels, it leads to a collapsed text field height being too small to fit the input text vertically.
The following screenshot shows a default collapsed M3 TextField on macOS. On M3 the font style is 16px with a 1.5 height, so the input height is 24. The decoration height is 16 because of the visual density reduction (this results in the border being misplaced, some letters overflowing and the cursor overflowing).

![image](https://github.com/flutter/flutter/assets/840911/0c854510-9d10-40a7-9a7e-8aa109f418e2)

After this PR, the minimum height is the input height.

![image](https://github.com/flutter/flutter/assets/840911/fcc67270-fd19-46ed-a2c2-55406f953e97)

## Related Issue

Fixes https://github.com/flutter/flutter/issues/150763

## Tests

Adds 4 tests, updates 2.

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

@@ -859,6 +859,8 @@ class _RenderDecoration extends RenderBox with SlottedContainerRenderObjectMixin
     return !decoration.isCollapsed && decoration.border.isOutline;
   }
 
+  Offset get _densityOffset => decoration.visualDensity.baseSizeAdjustment;
+
   @override
   void visitChildrenForSemantics(RenderObjectVisitor visitor) {
     if (icon != null) {
@@ -1025,9 +1027,8 @@ class _RenderDecoration extends RenderBox with SlottedContainerRenderObjectMixin
     // The height of the input needs to accommodate label above and counter and
     // helperError below, when they exist.
     final double bottomHeight = subtextSize?.bottomHeight ?? 0.0;
-    final Offset densityOffset = decoration.visualDensity.baseSizeAdjustment;
     final BoxConstraints inputConstraints = boxConstraints
-      .deflate(EdgeInsets.only(top: contentPadding.vertical + topHeight + bottomHeight + densityOffset.dy))
+      .deflate(EdgeInsets.only(top: contentPadding.vertical + topHeight + bottomHeight + _densityOffset.dy))
       .tighten(width: inputWidth);
 
     final RenderBox? input = this.input;
@@ -1067,10 +1068,10 @@ class _RenderDecoration extends RenderBox with SlottedContainerRenderObjectMixin
       + inputHeight
       + fixBelowInput
       + contentPadding.bottom
-      + densityOffset.dy,
+      + _densityOffset.dy,
     );
     final double minContainerHeight = decoration.isDense! || decoration.isCollapsed || expands
-      ? 0.0
+      ? inputHeight
       : kMinInteractiveDimension;
     final double maxContainerHeight = math.max(0.0, boxConstraints.maxHeight - bottomHeight);
     final double containerHeight = expands
@@ -1101,8 +1102,8 @@ class _RenderDecoration extends RenderBox with SlottedContainerRenderObjectMixin
       + inputInternalBaseline
       + baselineAdjustment
       + interactiveAdjustment
-      + densityOffset.dy / 2.0;
-    final double maxContentHeight = containerHeight - contentPadding.vertical - topHeight - densityOffset.dy;
+      + _densityOffset.dy / 2.0;
+    final double maxContentHeight = containerHeight - contentPadding.vertical - topHeight - _densityOffset.dy;
     final double alignableHeight = fixAboveInput + inputHeight + fixBelowInput;
     final double maxVerticalOffset = maxContentHeight - alignableHeight;
 
@@ -1234,12 +1235,11 @@ class _RenderDecoration extends RenderBox with SlottedContainerRenderObjectMixin
     final double inputHeight = _lineHeight(availableInputWidth, <RenderBox?>[input, hint]);
     final double inputMaxHeight = <double>[inputHeight, prefixHeight, suffixHeight].reduce(math.max);
 
-    final Offset densityOffset = decoration.visualDensity.baseSizeAdjustment;
     final double contentHeight = contentPadding.top
       + (label == null ? 0.0 : decoration.floatingLabelHeight)
       + inputMaxHeight
       + contentPadding.bottom
-      + densityOffset.dy;
+      + _densityOffset.dy;
     final double containerHeight = <double>[iconHeight, contentHeight, prefixIconHeight, suffixIconHeight].reduce(math.max);
     final double minContainerHeight = decoration.isDense! || expands
       ? 0.0
@@ -1491,8 +1491,7 @@ class _RenderDecoration extends RenderBox with SlottedContainerRenderObjectMixin
       // Temporary opt-in fix for https://github.com/flutter/flutter/issues/54028
       // Center the scaled label relative to the border.
       final double outlinedFloatingY = (-labelHeight * _kFinalLabelScale) / 2.0 + borderWeight / 2.0;
-      final Offset densityOffset = decoration.visualDensity.baseSizeAdjustment;
-      final double floatingY = isOutlineBorder ? outlinedFloatingY : contentPadding.top + densityOffset.dy / 2;
+      final double floatingY = isOutlineBorder ? outlinedFloatingY : contentPadding.top + _densityOffset.dy / 2;
       final double scale = lerpDouble(1.0, _kFinalLabelScale, t)!;
       final double centeredFloatX = _boxParentData(container!).offset.dx +
           _boxSize(container).width / 2.0 - floatWidth / 2.0;

packages/flutter/test/material/input_decorator_test.dart

@@ -6463,6 +6463,120 @@ void main() {
     });
   });
 
+  group('Material3 - InputDecoration collapsed', () {
+    // Overall height for a collapsed InputDecorator is 24dp which is the input
+    // height (font size = 16, line height = 1.5).
+    const double inputHeight = 24.0;
+
+    testWidgets('Decoration height is set to input height on mobile', (WidgetTester tester) async {
+      await tester.pumpWidget(
+        buildInputDecorator(
+          decoration: const InputDecoration.collapsed(
+            hintText: hintText,
+          ),
+        ),
+      );
+
+      expect(getDecoratorRect(tester).size, const Size(800.0, inputHeight));
+      expect(getInputRect(tester).height, inputHeight);
+      expect(getInputRect(tester).top, 0.0);
+      expect(getHintOpacity(tester), 0.0);
+
+      // The hint should appear.
+      await tester.pumpWidget(
+        buildInputDecorator(
+          isEmpty: true,
+          isFocused: true,
+          decoration: const InputDecoration.collapsed(
+            hintText: hintText,
+          ),
+        ),
+      );
+      await tester.pumpAndSettle();
+
+      expect(getDecoratorRect(tester).size, const Size(800.0, inputHeight));
+      expect(getInputRect(tester).height, inputHeight);
+      expect(getInputRect(tester).top, 0.0);
+      expect(getHintOpacity(tester), 1.0);
+      expect(getHintRect(tester).height, inputHeight);
+      expect(getHintRect(tester).top, 0.0);
+    }, variant: TargetPlatformVariant.mobile());
+
+    testWidgets('Decoration height is set to input height on desktop', (WidgetTester tester) async {
+      // Regression test for https://github.com/flutter/flutter/issues/150763.
+      await tester.pumpWidget(
+        buildInputDecorator(
+          decoration: const InputDecoration.collapsed(
+            hintText: hintText,
+          ),
+        ),
+      );
+
+      expect(getDecoratorRect(tester).size, const Size(800.0, inputHeight));
+      expect(getInputRect(tester).height, inputHeight);
+      expect(getInputRect(tester).top, 0.0);
+      expect(getHintOpacity(tester), 0.0);
+
+      // The hint should appear.
+      await tester.pumpWidget(
+        buildInputDecorator(
+          isEmpty: true,
+          isFocused: true,
+          decoration: const InputDecoration.collapsed(
+            hintText: hintText,
+          ),
+        ),
+      );
+      await tester.pumpAndSettle();
+
+      expect(getDecoratorRect(tester).size, const Size(800.0, inputHeight));
+      expect(getInputRect(tester).height, inputHeight);
+      expect(getInputRect(tester).top, 0.0);
+      expect(getHintOpacity(tester), 1.0);
+      expect(getHintRect(tester).height, inputHeight);
+      expect(getHintRect(tester).top, 0.0);
+    }, variant: TargetPlatformVariant.desktop());
+
+    testWidgets('InputDecoration.collapsed defaults to no border', (WidgetTester tester) async {
+      await tester.pumpWidget(
+        buildInputDecorator(
+          decoration: const InputDecoration.collapsed(
+            hintText: hintText,
+          ),
+        ),
+      );
+
+      expect(getBorderWeight(tester), 0.0);
+    });
+
+    test('InputDecorationTheme.isCollapsed is applied', () {
+      final InputDecoration decoration = const InputDecoration(
+        hintText: 'Hello, Flutter!',
+      ).applyDefaults(const InputDecorationTheme(
+        isCollapsed: true,
+      ));
+
+      expect(decoration.isCollapsed, true);
+    });
+
+    test('InputDecorationTheme.isCollapsed defaults to false', () {
+      final InputDecoration decoration = const InputDecoration(
+        hintText: 'Hello, Flutter!',
+      ).applyDefaults(const InputDecorationTheme());
+
+      expect(decoration.isCollapsed, false);
+    });
+
+    test('InputDecorationTheme.isCollapsed can be overriden', () {
+      final InputDecoration decoration = const InputDecoration(
+        isCollapsed: true,
+        hintText: 'Hello, Flutter!',
+      ).applyDefaults(const InputDecorationTheme());
+
+      expect(decoration.isCollapsed, true);
+    });
+  });
+
   testWidgets('InputDecorator counter text, widget, and null', (WidgetTester tester) async {
     Widget buildFrame({
       InputCounterWidgetBuilder? buildCounter,
@@ -7482,27 +7596,6 @@ void main() {
     expect(tester.takeException(), isNull);
   });
 
-  group('isCollapsed parameter works with themeData', () {
-    test('parameter is provided in InputDecorationTheme', () {
-      final InputDecoration decoration = const InputDecoration(
-        hintText: 'Hello, Flutter!',
-      ).applyDefaults(const InputDecorationTheme(
-          isCollapsed: true,
-      ));
-
-      expect(decoration.isCollapsed, true);
-    });
-
-    test('parameter is provided in InputDecoration', () {
-      final InputDecoration decoration = const InputDecoration(
-        isCollapsed: true,
-        hintText: 'Hello, Flutter!',
-      ).applyDefaults(const InputDecorationTheme());
-
-      expect(decoration.isCollapsed, true);
-    });
-  });
-
   testWidgets('Ensure the height of labelStyle remains unchanged when TextField is focused', (WidgetTester tester) async {
     // Regression test for https://github.com/flutter/flutter/issues/141448.
     final FocusNode focusNode = FocusNode();