Prevent dropdown menu's scroll offset from going negative (#22235)
In long lists this resulted in the dropdown scrolling to the very last
item in its list. Now clamping the value at `0.0`. Added a test to
verify that the selected item aligns with the button to test the offset.
Fixes flutter/flutter#15346
diff --git a/packages/flutter/lib/src/material/dropdown.dart b/packages/flutter/lib/src/material/dropdown.dart
index a651ee4..6fd94ea 100644
--- a/packages/flutter/lib/src/material/dropdown.dart
+++ b/packages/flutter/lib/src/material/dropdown.dart
@@ -346,9 +346,8 @@
}
if (scrollController == null) {
- double scrollOffset = 0.0;
- if (preferredMenuHeight > maxMenuHeight)
- scrollOffset = selectedItemOffset - (buttonTop - menuTop);
+ final double scrollOffset = (preferredMenuHeight > maxMenuHeight) ?
+ math.max(0.0, selectedItemOffset - (buttonTop - menuTop)) : 0.0;
scrollController = ScrollController(initialScrollOffset: scrollOffset);
}
diff --git a/packages/flutter/test/material/dropdown_test.dart b/packages/flutter/test/material/dropdown_test.dart
index d499caa..a5fa903 100644
--- a/packages/flutter/test/material/dropdown_test.dart
+++ b/packages/flutter/test/material/dropdown_test.dart
@@ -422,6 +422,57 @@
checkSelectedItemTextGeometry(tester, 'two');
});
+ testWidgets('Dropdown menu scrolls to first item in long lists', (WidgetTester tester) async {
+ // Open the dropdown menu
+ final Key buttonKey = UniqueKey();
+ await tester.pumpWidget(buildFrame(
+ buttonKey: buttonKey,
+ value: null, // nothing selected
+ items: List<String>.generate(/*length=*/ 100, (int index) => index.toString())
+ ));
+ await tester.tap(find.byKey(buttonKey));
+ await tester.pump();
+ await tester.pumpAndSettle(); // finish the menu animation
+
+ // Find the first item in the scrollable dropdown list
+ final Finder menuItemFinder = find.byType(Scrollable);
+ final RenderBox menuItemContainer = tester.renderObject<RenderBox>(menuItemFinder);
+ final RenderBox firstItem = tester.renderObject<RenderBox>(
+ find.descendant(of: menuItemFinder, matching: find.byKey(const ValueKey<String>('0'))));
+
+ // List should be scrolled so that the first item is at the top. Menu items
+ // are offset 8.0 from the top edge of the scrollable menu.
+ const Offset selectedItemOffset = Offset(0.0, -8.0);
+ expect(
+ firstItem.size.topCenter(firstItem.localToGlobal(selectedItemOffset)).dy,
+ equals(menuItemContainer.size.topCenter(menuItemContainer.localToGlobal(Offset.zero)).dy)
+ );
+ });
+
+ testWidgets('Dropdown menu aligns selected item with button in long lists', (WidgetTester tester) async {
+ // Open the dropdown menu
+ final Key buttonKey = UniqueKey();
+ await tester.pumpWidget(buildFrame(
+ buttonKey: buttonKey,
+ value: '50',
+ items: List<String>.generate(/*length=*/ 100, (int index) => index.toString())
+ ));
+ final RenderBox buttonBox = tester.renderObject(find.byKey(buttonKey));
+ await tester.tap(find.byKey(buttonKey));
+ await tester.pumpAndSettle(); // finish the menu animation
+
+ // Find the selected item in the scrollable dropdown list
+ final RenderBox selectedItem = tester.renderObject<RenderBox>(
+ find.descendant(of: find.byType(Scrollable), matching: find.byKey(const ValueKey<String>('50'))));
+
+ // List should be scrolled so that the selected item is in line with the button
+ expect(
+ selectedItem.size.center(selectedItem.localToGlobal(Offset.zero)).dy,
+ equals(buttonBox.size.center(buttonBox.localToGlobal(Offset.zero)).dy)
+ );
+ });
+
+
testWidgets('Size of DropdownButton with null value', (WidgetTester tester) async {
final Key buttonKey = UniqueKey();
String value;
