Ensure that the DropdownButton menu respects its parents bounds (#28371)

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

@@ -327,19 +327,67 @@ class _DropdownRoute<T> extends PopupRoute<_DropdownRouteResult<T>> {
 
   @override
   Widget buildPage(BuildContext context, Animation<double> animation, Animation<double> secondaryAnimation) {
+    return LayoutBuilder(
+      builder: (BuildContext context, BoxConstraints constraints) {
+        return _DropdownRoutePage<T>(
+          route: this,
+          constraints: constraints,
+          items: items,
+          padding: padding,
+          buttonRect: buttonRect,
+          selectedIndex: selectedIndex,
+          elevation: elevation,
+          theme: theme,
+          style: style,
+        );
+      }
+    );
+  }
+
+  void _dismiss() {
+    navigator?.removeRoute(this);
+  }
+}
+
+class _DropdownRoutePage<T> extends StatelessWidget {
+ const  _DropdownRoutePage({
+    Key key,
+    this.route,
+    this.constraints,
+    this.items,
+    this.padding,
+    this.buttonRect,
+    this.selectedIndex,
+    this.elevation = 8,
+    this.theme,
+    this.style,
+  }) : super(key: key);
+
+  final _DropdownRoute<T> route;
+  final BoxConstraints constraints;
+  final List<DropdownMenuItem<T>> items;
+  final EdgeInsetsGeometry padding;
+  final Rect buttonRect;
+  final int selectedIndex;
+  final int elevation;
+  final ThemeData theme;
+  final TextStyle style;
+
+  @override
+  Widget build(BuildContext context) {
     assert(debugCheckHasDirectionality(context));
-    final double screenHeight = MediaQuery.of(context).size.height;
-    final double maxMenuHeight = screenHeight - 2.0 * _kMenuItemHeight;
+    final double availableHeight = constraints.maxHeight;
+    final double maxMenuHeight = availableHeight - 2.0 * _kMenuItemHeight;
 
     final double buttonTop = buttonRect.top;
-    final double buttonBottom = buttonRect.bottom;
+    final double buttonBottom = math.min(buttonRect.bottom, availableHeight);
 
     // If the button is placed on the bottom or top of the screen, its top or
     // bottom may be less than [_kMenuItemHeight] from the edge of the screen.
     // In this case, we want to change the menu limits to align with the top
     // or bottom edge of the button.
     final double topLimit = math.min(_kMenuItemHeight, buttonTop);
-    final double bottomLimit = math.max(screenHeight - _kMenuItemHeight, buttonBottom);
+    final double bottomLimit = math.max(availableHeight - _kMenuItemHeight, buttonBottom);
 
     final double selectedItemOffset = selectedIndex * _kMenuItemHeight + kMaterialListPadding.top;
 
@@ -359,24 +407,25 @@ class _DropdownRoute<T> extends PopupRoute<_DropdownRouteResult<T>> {
     // respectively.
     if (menuTop < topLimit)
       menuTop = math.min(buttonTop, topLimit);
+
     if (menuBottom > bottomLimit) {
       menuBottom = math.max(buttonBottom, bottomLimit);
       menuTop = menuBottom - menuHeight;
     }
 
-    if (scrollController == null) {
+    if (route.scrollController == null) {
       // The limit is asymmetrical because we do not care how far positive the
       // limit goes. We are only concerned about the case where the value of
       // [buttonTop - menuTop] is larger than selectedItemOffset, ie. when
       // the button is close to the bottom of the screen and the selected item
       // is close to 0.
       final double scrollOffset = preferredMenuHeight > maxMenuHeight ? math.max(0.0, selectedItemOffset - (buttonTop - menuTop)) : 0.0;
-      scrollController = ScrollController(initialScrollOffset: scrollOffset);
+      route.scrollController = ScrollController(initialScrollOffset: scrollOffset);
     }
 
     final TextDirection textDirection = Directionality.of(context);
     Widget menu = _DropdownMenu<T>(
-      route: this,
+      route: route,
       padding: padding.resolve(textDirection),
     );
 
@@ -404,10 +453,6 @@ class _DropdownRoute<T> extends PopupRoute<_DropdownRouteResult<T>> {
       ),
     );
   }
-
-  void _dismiss() {
-    navigator?.removeRoute(this);
-  }
 }
 
 /// An item in a menu created by a [DropdownButton].

packages/flutter/test/material/dropdown_test.dart

@@ -1035,4 +1035,49 @@ void main() {
     await tester.pumpAndSettle();
     expect(getMenuScroll(), 4312.0);
   });
+
+  testWidgets('Dropdown menu respects parent size limits', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/24417
+
+    int selectedIndex;
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          bottomNavigationBar: const SizedBox(height: 200),
+          body: Navigator(
+            onGenerateRoute: (RouteSettings settings) {
+              return MaterialPageRoute<void>(
+                builder: (BuildContext context) {
+                  return SafeArea(
+                    child: Container(
+                      alignment: Alignment.topLeft,
+                      // From material/dropdown.dart (menus are unaligned by default):
+                      //  _kUnalignedMenuMargin = EdgeInsetsDirectional.only(start: 16.0, end: 24.0)
+                      // This padding ensures that the entire menu will be visible
+                      padding: const EdgeInsetsDirectional.only(start: 16.0, end: 24.0),
+                      child: DropdownButton<int>(
+                        value: 12,
+                        onChanged: (int i) { selectedIndex = i; },
+                        items: List<DropdownMenuItem<int>>.generate(100, (int i) {
+                          return DropdownMenuItem<int>(value: i, child: Text('$i'));
+                        }),
+                      ),
+                    ),
+                  );
+                },
+              );
+            },
+          ),
+        ),
+      ),
+    );
+
+    await tester.tap(find.text('12'));
+    await tester.pumpAndSettle();
+    expect(selectedIndex, null);
+
+    await tester.tap(find.text('13').last);
+    await tester.pumpAndSettle();
+    expect(selectedIndex, 13);
+  });
 }