[material/dropdown_menu.dart] Refactor _RenderDropdownMenuBody.computeDryLayout (#176503)

_RenderDropdownMenuBody.computeDryLayout accesses this.constraints, but
it should only access the constraints parameter passed into
computeDryLayout. This PR removes this.constraints access.

<img width="621" height="73" alt="image"
src="https://github.com/user-attachments/assets/495573e3-3b91-4091-8a7c-76594c98e22f"
/>

Also, I removed a line in which the child's offset is being set in
computeDryLayout. This appears to be an error:
<img width="402" height="56" alt="image"
src="https://github.com/user-attachments/assets/0045761d-c958-451b-a6ec-cbdf0fe7bd09"
/>

Finally, I'm curious whether there is a reason why only the first child
is being used to set the height?

Resolves flutter/flutter#176494

Blocking flutter/flutter#176494

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Author: davidhicks980

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

@@ -1448,7 +1448,6 @@ class _RenderDropdownMenuBody extends RenderBox
 
   @override
   Size computeDryLayout(BoxConstraints constraints) {
-    final BoxConstraints constraints = this.constraints;
     double maxWidth = 0.0;
     double? maxHeight;
     RenderBox? child = firstChild;
@@ -1460,22 +1459,17 @@ class _RenderDropdownMenuBody extends RenderBox
     );
 
     while (child != null) {
-      if (child == firstChild) {
-        final Size childSize = child.getDryLayout(innerConstraints);
-        maxHeight ??= childSize.height;
-        final _DropdownMenuBodyParentData childParentData =
-            child.parentData! as _DropdownMenuBodyParentData;
-        assert(child.parentData == childParentData);
-        child = childParentData.nextSibling;
-        continue;
-      }
       final Size childSize = child.getDryLayout(innerConstraints);
+
+      // The first child is the TextField, which doesn't contribute to the
+      // menu's width calculation.
+      if (child != firstChild) {
+        maxWidth = math.max(maxWidth, childSize.width);
+      }
+
       final _DropdownMenuBodyParentData childParentData =
           child.parentData! as _DropdownMenuBodyParentData;
-      childParentData.offset = Offset.zero;
-      maxWidth = math.max(maxWidth, childSize.width);
       maxHeight ??= childSize.height;
-      assert(child.parentData == childParentData);
       child = childParentData.nextSibling;
     }
 

packages/flutter/test/material/dropdown_menu_test.dart

@@ -953,6 +953,32 @@ void main() {
     expect(dropdownWidth, menuWidth);
   });
 
+  // Regression test for https://github.com/flutter/flutter/issues/176501
+  testWidgets('_RenderDropdownMenuBody.computeDryLayout does not access this.constraints', (
+    WidgetTester tester,
+  ) async {
+    await tester.pumpWidget(
+      const MaterialApp(
+        home: Scaffold(
+          body: Center(
+            child: _TestDryLayout(
+              child: DropdownMenu<int>(
+                dropdownMenuEntries: <DropdownMenuEntry<int>>[
+                  DropdownMenuEntry<int>(value: 1, label: 'One'),
+                  DropdownMenuEntry<int>(value: 2, label: 'Two'),
+                ],
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+
+    // The test passes if no exception is thrown during the layout phase.
+    expect(tester.takeException(), isNull);
+    expect(find.byType(DropdownMenu<int>), findsOneWidget);
+  });
+
   testWidgets(
     'Material2 - The menuHeight property can be used to show a shorter scrollable menu list instead of the complete list',
     (WidgetTester tester) async {
@@ -4787,3 +4813,29 @@ enum ShortMenu {
   const ShortMenu(this.label);
   final String label;
 }
+
+// A helper widget that creates a render object designed to call `getDryLayout`
+// on its child during its own `performLayout` phase. This is used to test
+// that a child's `computeDryLayout` implementation is valid.
+class _TestDryLayout extends SingleChildRenderObjectWidget {
+  const _TestDryLayout({super.child});
+
+  @override
+  RenderObject createRenderObject(BuildContext context) {
+    return _RenderTestDryLayout();
+  }
+}
+
+class _RenderTestDryLayout extends RenderProxyBox {
+  @override
+  void performLayout() {
+    if (child == null) {
+      size = constraints.smallest;
+      return;
+    }
+
+    child!.getDryLayout(constraints);
+    child!.layout(constraints, parentUsesSize: true);
+    size = child!.size;
+  }
+}