Fix last month not being displayed if last date is selected in DateRangePicker (#66542)

packages/flutter/lib/src/material/pickers/calendar_date_range_picker.dart

@@ -109,8 +109,8 @@ class _CalendarDateRangePickerState extends State<CalendarDateRangePicker> {
     // Calculate the index for the initially displayed month. This is needed to
     // divide the list of months into two `SliverList`s.
     final DateTime initialDate = widget.initialStartDate ?? widget.currentDate;
-    if (widget.firstDate.isBefore(initialDate) &&
-        widget.lastDate.isAfter(initialDate)) {
+    if (!initialDate.isBefore(widget.firstDate) &&
+        !initialDate.isAfter(widget.lastDate)) {
       _initialMonthIndex = utils.monthDelta(widget.firstDate, initialDate);
     }
 

packages/flutter/test/material/date_range_picker_test.dart

@@ -13,6 +13,7 @@ import 'feedback_tester.dart';
 void main() {
   DateTime firstDate;
   DateTime lastDate;
+  DateTime currentDate;
   DateTimeRange initialDateRange;
   DatePickerEntryMode initialEntryMode = DatePickerEntryMode.calendar;
 
@@ -31,6 +32,7 @@ void main() {
   setUp(() {
     firstDate = DateTime(2015, DateTime.january, 1);
     lastDate = DateTime(2016, DateTime.december, 31);
+    currentDate = null;
     initialDateRange = DateTimeRange(
       start: DateTime(2016, DateTime.january, 15),
       end: DateTime(2016, DateTime.january, 25),
@@ -79,6 +81,7 @@ void main() {
       initialDateRange: initialDateRange,
       firstDate: firstDate,
       lastDate: lastDate,
+      currentDate: currentDate,
       initialEntryMode: initialEntryMode,
       cancelText: cancelText,
       confirmText: confirmText,
@@ -122,6 +125,52 @@ void main() {
     });
   });
 
+  testWidgets('Last month header should be visible if last date is selected',
+      (WidgetTester tester) async {
+    firstDate = DateTime(2015, DateTime.january, 1);
+    lastDate = DateTime(2016, DateTime.december, 31);
+    initialDateRange = DateTimeRange(
+      start: lastDate,
+      end: lastDate,
+    );
+    await preparePicker(tester, (Future<DateTimeRange> range) async {
+      // December header should be showing, but no November
+      expect(find.text('December 2016'), findsOneWidget);
+      expect(find.text('November 2016'), findsNothing);
+    });
+  });
+
+  testWidgets('First month header should be visible if first date is selected',
+      (WidgetTester tester) async {
+    firstDate = DateTime(2015, DateTime.january, 1);
+    lastDate = DateTime(2016, DateTime.december, 31);
+    initialDateRange = DateTimeRange(
+      start: firstDate,
+      end: firstDate,
+    );
+    await preparePicker(tester, (Future<DateTimeRange> range) async {
+      // January and February headers should be showing, but no March
+      expect(find.text('January 2015'), findsOneWidget);
+      expect(find.text('February 2015'), findsOneWidget);
+      expect(find.text('March 2015'), findsNothing);
+    });
+  });
+
+  testWidgets('Current month header should be visible if no date is selected',
+      (WidgetTester tester) async {
+    firstDate = DateTime(2015, DateTime.january, 1);
+    lastDate = DateTime(2016, DateTime.december, 31);
+    currentDate = DateTime(2016, DateTime.september, 1);
+    initialDateRange = null;
+
+    await preparePicker(tester, (Future<DateTimeRange> range) async {
+      // September and October headers should be showing, but no August
+      expect(find.text('September 2016'), findsOneWidget);
+      expect(find.text('October 2016'), findsOneWidget);
+      expect(find.text('August 2016'), findsNothing);
+    });
+  });
+
   testWidgets('Can cancel', (WidgetTester tester) async {
     await preparePicker(tester, (Future<DateTimeRange> range) async {
       await tester.tap(find.byIcon(Icons.close));
@@ -372,10 +421,10 @@ void main() {
 
     testWidgets('Navigating with arrow keys scrolls as needed', (WidgetTester tester) async {
       await preparePicker(tester, (Future<DateTimeRange> range) async {
-        // Jan and Feb headers should be showing, but no Mar
+        // Jan and Feb headers should be showing, but no March
         expect(find.text('January 2016'), findsOneWidget);
         expect(find.text('February 2016'), findsOneWidget);
-        expect(find.text('Mar 2016'), findsNothing);
+        expect(find.text('March 2016'), findsNothing);
 
         // Navigate to the grid
         await tester.sendKeyEvent(LogicalKeyboardKey.tab);