Fix `InputDecorator` child vertical alignment (#112003)

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

@@ -1114,15 +1114,13 @@ class _RenderDecoration extends RenderBox with SlottedContainerRenderObjectMixin
       + topHeight
       + inputInternalBaseline
       + baselineAdjustment
-      + interactiveAdjustment;
-    final double maxContentHeight = containerHeight
-      - contentPadding.top
-      - topHeight
-      - contentPadding.bottom;
+      + interactiveAdjustment
+      + densityOffset.dy / 2.0;
+    final double maxContentHeight = containerHeight - contentPadding.vertical - topHeight - densityOffset.dy;
     final double alignableHeight = fixAboveInput + inputHeight + fixBelowInput;
     final double maxVerticalOffset = maxContentHeight - alignableHeight;
     final double textAlignVerticalOffset = maxVerticalOffset * textAlignVerticalFactor;
-    final double inputBaseline = topInputBaseline + textAlignVerticalOffset + densityOffset.dy / 2.0;
+    final double inputBaseline = topInputBaseline + textAlignVerticalOffset;
 
     // The three main alignments for the baseline when an outline is present are
     //
@@ -1302,7 +1300,7 @@ class _RenderDecoration extends RenderBox with SlottedContainerRenderObjectMixin
 
   @override
   double computeDistanceToActualBaseline(TextBaseline baseline) {
-    return _boxParentData(input!).offset.dy + input!.computeDistanceToActualBaseline(baseline)!;
+    return _boxParentData(input!).offset.dy + (input?.computeDistanceToActualBaseline(baseline) ?? 0.0);
   }
 
   // Records where the label was painted.
@@ -1325,12 +1323,13 @@ class _RenderDecoration extends RenderBox with SlottedContainerRenderObjectMixin
     final double overallWidth = constraints.maxWidth;
     final double overallHeight = layout.containerHeight + layout.subtextHeight;
 
+    final RenderBox? container = this.container;
     if (container != null) {
       final BoxConstraints containerConstraints = BoxConstraints.tightFor(
         height: layout.containerHeight,
         width: overallWidth - _boxSize(icon).width,
       );
-      container!.layout(containerConstraints, parentUsesSize: true);
+      container.layout(containerConstraints, parentUsesSize: true);
       final double x;
       switch (textDirection) {
         case TextDirection.rtl:
@@ -1340,7 +1339,7 @@ class _RenderDecoration extends RenderBox with SlottedContainerRenderObjectMixin
           x = _boxSize(icon).width;
           break;
        }
-      _boxParentData(container!).offset = Offset(x, 0.0);
+      _boxParentData(container).offset = Offset(x, 0.0);
     }
 
     late double height;

packages/flutter/test/material/input_decorator_test.dart

@@ -2290,6 +2290,54 @@ void main() {
         // In between the center and bottom aligned cases.
         expect(tester.getTopLeft(find.text(text)).dy, moreOrLessEquals(useMaterial3 ? 497.9375 : 498.5, epsilon: .0001));
       });
+
+      testWidgets('works with density and content padding', (WidgetTester tester) async {
+        const Key key = Key('child');
+        const Key containerKey = Key('container');
+        const double totalHeight = 100.0;
+        const double childHeight = 20.0;
+        const VisualDensity visualDensity = VisualDensity(vertical: VisualDensity.maximumDensity);
+        const EdgeInsets contentPadding = EdgeInsets.only(top: 6, bottom: 14);
+
+        await tester.pumpWidget(
+          Center(
+            child: SizedBox(
+              key: containerKey,
+              height: totalHeight,
+              child: buildInputDecorator(
+                useMaterial3: useMaterial3,
+                // isEmpty: false (default)
+                // isFocused: false (default)
+                expands: true,
+                decoration: const InputDecoration(
+                  border: InputBorder.none,
+                  contentPadding: contentPadding,
+                ),
+                textAlignVertical: TextAlignVertical.center,
+                visualDensity: visualDensity,
+                child: const SizedBox(key: key, height: childHeight),
+              ),
+            ),
+          ),
+        );
+
+        // Vertical components: contentPadding.vertical, densityOffset.y, child
+        final double childVerticalSpaceAffordance = totalHeight
+                                                  - visualDensity.baseSizeAdjustment.dy
+                                                  - contentPadding.vertical;
+
+        // TextAlignVertical.center is specified so `child` needs to be centered
+        // in the avaiable space.
+        final double childMargin = (childVerticalSpaceAffordance - childHeight) / 2;
+        final double childTop = visualDensity.baseSizeAdjustment.dy / 2.0
+                              + contentPadding.top
+                              + childMargin;
+
+        expect(
+          tester.getTopLeft(find.byKey(key)).dy,
+          tester.getTopLeft(find.byKey(containerKey)).dy + childTop,
+        );
+      });
     });
 
     group('outline border', () {