SfCalendar: LateInitializationError '_scrollStartPosition' when viewNavigationMode changes from none to snap during an in-flight drag

[oooooocean]

Bug description

_CustomCalendarScrollViewState._scrollStartPosition (lib/src/calendar/views/calendar_view.dart) is declared late double and is only assigned in the ViewNavigationMode.snap branch of _onHorizontalStart / _onVerticalStart — the ViewNavigationMode.none branch returns without assigning. _onHorizontalUpdate / _onVerticalUpdate read the field unguarded in their snap branch.

If the host app changes SfCalendar.viewNavigationMode from none to snap while a view-swipe drag is in flight (the calendar widget rebuilds, but the internal State is preserved), the same drag's next update callback takes the snap branch and reads the never-assigned late field:

LateInitializationError: Field '_scrollStartPosition' has not been initialized.

This is a production fatal crash for us (Crashlytics, Android, syncfusion_flutter_calendar 33.2.8). The field and guard structure are unchanged on current master (34.x), so the latest version is affected as well.

Note that toggling viewNavigationMode at runtime is sanctioned usage — KB article 10950 ("How to restrict the view navigation in the Flutter Calendar", https://support.syncfusion.com/kb/article/10950) demonstrates exactly this pattern. Our app disables page navigation during a pinch-zoom interaction and re-enables it on a short timer; when the timer fires while the user is still dragging, the crash reproduces.

Steps to reproduce

Deterministic widget-test reproduction (crashes on 33.2.8 and on master):

import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';
import 'package:syncfusion_flutter_calendar/calendar.dart';

void main() {
  testWidgets('viewNavigationMode none→snap flip mid-drag crashes', (tester) async {
    Widget host(ViewNavigationMode mode) => MaterialApp(
          home: Scaffold(
            body: SfCalendar(view: CalendarView.day, viewNavigationMode: mode),
          ),
        );

    await tester.pumpWidget(host(ViewNavigationMode.none));
    await tester.pumpAndSettle();

    // Drag starts while navigation is disabled: _onHorizontalStart's `none`
    // branch returns without recording _scrollStartPosition.
    final gesture = await tester.startGesture(tester.getCenter(find.byType(SfCalendar)));
    await gesture.moveBy(const Offset(-40, 0));
    await tester.pump();

    // Host re-enables navigation mid-drag (same State — no key change).
    await tester.pumpWidget(host(ViewNavigationMode.snap));
    await tester.pump();

    // The same in-flight drag now takes the snap branch in _onHorizontalUpdate
    // and reads the uninitialized `late` field → LateInitializationError.
    await gesture.moveBy(const Offset(-40, 0));
    await tester.pump();
    await gesture.up();
    await tester.pumpAndSettle();
  });
}

The vertical axis (CalendarView.month + MonthNavigationDirection.vertical, _onVerticalUpdate) reproduces the same way.

Expected behavior

The drag survives the mode change without crashing (e.g. treat the first post-flip update as the drag baseline).

Suggested fix

Make the field nullable and seed it on first read in the update handlers, resetting at drag end:

double? _scrollStartPosition;

// _onHorizontalUpdate (snap branch):
final double scrollStartPosition =
    _scrollStartPosition ??= dragUpdateDetails.globalPosition.dx;
final double difference =
    dragUpdateDetails.globalPosition.dx - scrollStartPosition;

// _onHorizontalEnd / _onVerticalEnd (entry):
_scrollStartPosition = null;

Seeding from the current pointer keeps difference == 0 on the flipped tick, so there is no crash and no spurious page turn from a zero/stale baseline. (Initializing the field to 0 instead would compute difference from the absolute screen coordinate and trigger an unintended page navigation.) Normal snap-only drags are behavior-identical since the start handler always assigns before the first update.

We are running this fix in production via a fork and are happy to provide more detail.

• Package: syncfusion_flutter_calendar 33.2.8 (also present on master / 34.x source)
• Platform: Android (mechanism is platform-independent)
• Flutter: 3.44.0

[Mugunthan-Ramalingam]

Hi oooooocean,

We have replicated the reported issue regarding the LateInitializationError that occurs when SfCalendar.viewNavigationMode is viewNavigationMode is changed from ViewNavigationMode.none to ViewNavigationMode.snap during an active swipe gesture. The issue is scheduled to be fixed in our upcoming weekly patch release on July 21, 2026. We will notify you here once the release has been rolled out, and we appreciate your patience in the meantime.

Root cause:
The _scrollStartPosition field in _CustomCalendarScrollViewState is only initialized when a drag gesture starts with ViewNavigationMode.none. However, if the navigation mode changes to snap while the drag is still in progress, the drag update callbacks try to read this uninitialized field, causing a LateInitializationError.

Regards,
Mugunthan.

[Mugunthan-Ramalingam]

Hi oooooocean,

The reported issue has been resolved, and the fix is included in the version mentioned below. We kindly request you to upgrade the syncfusion_flutter_calendar package to the latest version to avoid this issue.

Version: https://pub.dev/packages/syncfusion_flutter_calendar/versions/34.1.32

Root Cause:

The _scrollStartPosition field in _CustomCalendarScrollViewState is only initialized when a drag gesture starts with ViewNavigationMode.none. However, if the navigation mode changes to snap while the drag is still in progress, the drag update callbacks try to read this uninitialized field, causing a LateInitializationError.

Thank you for your patience and understanding. Please let us know if you need any assistance with the upgrade or have further questions.

Regards,
Mugunthan.