fix: reset monthSelectedIn when changeMonth is called in custom header

src/calendar.tsx

@@ -180,6 +180,7 @@ type CalendarProps = React.PropsWithChildren<
       showPreviousMonths?: boolean;
       monthsShown?: number;
       monthSelectedIn?: number;
+      onMonthSelectedInChange?: (monthSelectedIn: number) => void;
       onSelect: (
         day: Date,
         event?:
@@ -455,7 +456,12 @@ export default class Calendar extends Component<CalendarProps, CalendarState> {
       ({ date }) => ({
         date: setMonth(date, Number(month)),
       }),
-      () => this.handleMonthChange(this.state.date),
+      () => {
+        this.handleMonthChange(this.state.date);
+        // Reset monthSelectedIn to 0 so the target month appears in the leftmost position
+        // This ensures consistent behavior when using changeMonth in custom headers
+        this.props.onMonthSelectedInChange?.(0);
+      },
     );
   };
 

src/index.tsx

@@ -1463,6 +1463,10 @@ export class DatePicker extends Component<DatePickerProps, DatePickerState> {
     }
   };
 
+  handleMonthSelectedInChange = (monthSelectedIn: number): void => {
+    this.setState({ monthSelectedIn });
+  };
+
   renderCalendar = () => {
     if (!this.props.inline && !this.isCalendarOpen()) {
       return null;
@@ -1494,6 +1498,7 @@ export class DatePicker extends Component<DatePickerProps, DatePickerState> {
         dropdownMode={
           this.props.dropdownMode ?? DatePicker.defaultProps.dropdownMode
         }
+        onMonthSelectedInChange={this.handleMonthSelectedInChange}
       >
         {this.props.children}
       </Calendar>

src/test/calendar_test.test.tsx

@@ -984,6 +984,47 @@ describe("Calendar", () => {
       expect(header).toHaveLength(1);
       expect(time).toHaveLength(1);
     });
+
+    it("should display the target month in the leftmost position when changeMonth is called with monthsShown >= 2", () => {
+      // This test verifies the fix for issue #3829
+      // When using changeMonth in a custom header with monthsShown >= 2,
+      // the target month should always appear in the leftmost position
+      // regardless of which calendar panel the user last selected a date in
+      const onMonthSelectedInChangeSpy = jest.fn();
+
+      const renderCustomHeaderWithMonthSelect = ({
+        changeMonth,
+      }: {
+        changeMonth: (month: number) => void;
+      }) => (
+        <div className="custom-header">
+          <select
+            className="month-select"
+            onChange={({ target: { value } }) => changeMonth(Number(value))}
+          >
+            {months.map((option, index) => (
+              <option key={option} value={index}>
+                {option}
+              </option>
+            ))}
+          </select>
+        </div>
+      );
+
+      const { calendar } = getCalendar({
+        renderCustomHeader: renderCustomHeaderWithMonthSelect,
+        monthsShown: 2,
+        onMonthSelectedInChange: onMonthSelectedInChangeSpy,
+      });
+
+      // Select June (month index 5) from the month dropdown
+      const monthSelect = safeQuerySelector(calendar, ".month-select");
+      fireEvent.change(monthSelect, { target: { value: 5 } });
+
+      // Verify that onMonthSelectedInChange was called with 0
+      // This ensures the target month appears in the leftmost position
+      expect(onMonthSelectedInChangeSpy).toHaveBeenCalledWith(0);
+    });
   });
 
   describe("when showDisabledMonthNavigation is enabled", () => {

src/test/datepicker_test.test.tsx

@@ -2897,6 +2897,59 @@ describe("DatePicker", () => {
     expect(instance!.state.monthSelectedIn).toEqual(undefined);
   });
 
+  it("should reset monthSelectedIn to 0 when changeMonth is called from custom header", () => {
+    let instance: DatePicker | null = null;
+    let changeMonthFn: ((month: number) => void) | null = null;
+
+    const { container } = render(
+      <DatePicker
+        ref={(node) => {
+          instance = node;
+        }}
+        inline
+        monthsShown={2}
+        selected={newDate("2024-06-15")}
+        renderCustomHeader={({ changeMonth }) => {
+          changeMonthFn = changeMonth;
+          return (
+            <div>
+              <button
+                data-testid="change-month-btn"
+                onClick={() => changeMonth(0)}
+              >
+                Go to January
+              </button>
+            </div>
+          );
+        }}
+      />,
+    );
+
+    expect(instance).toBeTruthy();
+
+    // First, select a day in the second month panel to set monthSelectedIn to 1
+    const dayButtonsInSecondMonth = container
+      .querySelectorAll(".react-datepicker__month-container")[1]
+      ?.querySelectorAll(
+        ".react-datepicker__day:not(.react-datepicker__day--outside-month)",
+      );
+    expect(dayButtonsInSecondMonth).toBeTruthy();
+    expect(dayButtonsInSecondMonth!.length).toBeGreaterThan(0);
+
+    // Click a day in the second month to set monthSelectedIn to 1
+    fireEvent.click(dayButtonsInSecondMonth![10]!);
+    expect(instance!.state.monthSelectedIn).toEqual(1);
+
+    // Now call changeMonth from the custom header
+    expect(changeMonthFn).toBeTruthy();
+    act(() => {
+      changeMonthFn!(0); // Change to January
+    });
+
+    // monthSelectedIn should be reset to 0
+    expect(instance!.state.monthSelectedIn).toEqual(0);
+  });
+
   it("should show the popper arrow when showPopperArrow is true", () => {
     const { container } = render(<DatePicker showPopperArrow />);
     const input = safeQuerySelector(container, "input");