Fix DropdownButton crash when viewport size is too small for the entirety of...

Fix DropdownButton crash when viewport size is too small for the entirety of the menu's height (#76764)

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

@@ -65,12 +65,12 @@ class _DropdownMenuPainter extends CustomPainter {
   void paint(Canvas canvas, Size size) {
     final double selectedItemOffset = getSelectedItemOffset();
     final Tween<double> top = Tween<double>(
-      begin: selectedItemOffset.clamp(0.0, size.height - _kMenuItemHeight),
+      begin: selectedItemOffset.clamp(0.0, math.max(size.height - _kMenuItemHeight, 0.0)),
       end: 0.0,
     );
 
     final Tween<double> bottom = Tween<double>(
-      begin: (top.begin! + _kMenuItemHeight).clamp(_kMenuItemHeight, size.height),
+      begin: (top.begin! + _kMenuItemHeight).clamp(math.min(_kMenuItemHeight, size.height), size.height),
       end: size.height,
     );
 

packages/flutter/test/material/dropdown_test.dart

@@ -3008,4 +3008,61 @@ void main() {
     expect(scrollController.position.maxScrollExtent > 0, isTrue);
     expect(find.byType(Scrollbar), paints..rect());
   });
+
+  // Regression test for https://github.com/flutter/flutter/issues/76614
+  testWidgets('Do not crash if used in very short screen', (WidgetTester tester) async {
+    // The default item height is 48.0 pixels and needs two items padding since
+    // the menu requires empty space surrounding the menu. Finally, the constraint height
+    // is 47.0 pixels for the menu rendering.
+    tester.binding.window.physicalSizeTestValue = const Size(800.0, 48.0 * 3 - 1.0);
+    tester.binding.window.devicePixelRatioTestValue = 1;
+    addTearDown(tester.binding.window.clearPhysicalSizeTestValue);
+    addTearDown(tester.binding.window.clearDevicePixelRatioTestValue);
+
+    const String value = 'foo';
+    final UniqueKey itemKey = UniqueKey();
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          body: Center(
+            child: DropdownButton<String>(
+              value: value,
+              items: <DropdownMenuItem<String>>[
+                DropdownMenuItem<String>(
+                  key: itemKey,
+                  value: value,
+                  child: const Text(value),
+                ),
+              ],
+              onChanged: (_) { },
+            ),
+          ),
+        ),
+      ),
+    );
+
+    await tester.tap(find.text(value));
+    await tester.pumpAndSettle();
+
+    final List<RenderBox> itemBoxes = tester.renderObjectList<RenderBox>(find.byKey(itemKey)).toList();
+    expect(itemBoxes[0].localToGlobal(Offset.zero).dx, 364.0);
+    expect(itemBoxes[0].localToGlobal(Offset.zero).dy, 47.5);
+
+    expect(itemBoxes[1].localToGlobal(Offset.zero).dx, 364.0);
+    expect(itemBoxes[1].localToGlobal(Offset.zero).dy, 47.5);
+
+    expect(
+      find.ancestor(
+        of: find.text(value).last,
+        matching: find.byType(CustomPaint),
+      ).at(2),
+      paints
+        ..save()
+        ..rrect()
+        ..rrect()
+        ..rrect()
+        // The height of menu is 47.0.
+        ..rrect(rrect: const RRect.fromLTRBXY(0.0, 0.0, 112.0, 47.0, 2.0, 2.0), color: Colors.grey[50], hasMaskFilter: false)
+    );
+  });
 }