🐛 Fix scroll behavior by replacing ScrollController with ZoomScrollController and updating scroll listener parameters across day, multi-day, and week views

Author: kavantrivedi

doc/documentation.md

@@ -603,6 +603,64 @@ CalendarControllerProvider(
 
 When a view does not receive `controller` directly, it reads the controller from `CalendarControllerProvider`.
 
+## Advanced Internals: `ZoomScrollController` (Optional)
+
+`ZoomScrollController` is an internal utility used by `DayView`, `WeekView`, and
+`MultiDayView` to keep the same time range visible when `heightPerMinute`
+changes.
+
+> This is **not part of the stable public package API** exported by
+> `package:calendar_view/calendar_view.dart`.
+>
+> Use this only when building a custom calendar/scroll implementation and when
+> you are okay with potential breaking changes in internal files.
+
+## Why it exists
+
+When zoom changes (for example from `heightPerMinute: 1.0` to `1.4`), a normal
+`ScrollController` update done after build can produce a brief visual jump.
+`ZoomScrollController` avoids that by preparing the next offset before layout
+and applying it during content-dimension calculation.
+
+## Internal contract
+
+1. Read the current vertical offset.
+2. Compute scaled offset:
+   `scaledOffset = (currentOffset / oldHeightPerMinute) * newHeightPerMinute`
+3. Call `prepareZoomJump(scaledOffset)` before rebuild.
+4. Rebuild with new `heightPerMinute`.
+
+Because the correction is applied during layout, viewport size and scroll
+position update together in the same frame.
+
+## Minimal internal example
+
+```dart
+import 'package:calendar_view/src/zoom_scroll_controller.dart';
+
+class MyZoomState {
+  final controller = ZoomScrollController();
+  double heightPerMinute = 1.0;
+
+  void onZoomChange(double nextHeightPerMinute) {
+    final currentOffset = controller.hasClients ? controller.offset : 0.0;
+
+    final scaledOffset =
+        (currentOffset / heightPerMinute) * nextHeightPerMinute;
+
+    controller.prepareZoomJump(scaledOffset);
+    heightPerMinute = nextHeightPerMinute;
+  }
+}
+```
+
+## Recommendation
+
+Prefer the built-in `DayView`, `WeekView`, and `MultiDayView` behavior unless
+you are implementing a custom view layer that needs zoom-aware scroll
+correction.
+
+
 # Localization Guide
 
 This guide covers localization support in `calendar_view` and how to keep localized strings aligned with the package API.
@@ -1159,7 +1217,6 @@ dependencies:
      bool hideDaysNotInMonth,
    );
    ```
-
 # Contributors
 
 ## Main Contributors

example/lib/widgets/day_view_widget.dart

@@ -22,6 +22,7 @@ class DayViewWidget extends StatelessWidget {
       timeLineBuilder: (date) => _timeLineBuilder(date, isLtr),
       scrollPhysics: const BouncingScrollPhysics(),
       eventArranger: SideEventArranger(),
+      keepScrollOffset: true,
       showQuarterHours: false,
       showMidnightHour: true,
       hourIndicatorSettings: HourIndicatorSettings(

example/lib/widgets/week_view_widget.dart

@@ -24,6 +24,7 @@ class WeekViewWidget extends StatelessWidget {
       showWeekends: true,
       showMidnightHour: true,
       showLiveTimeLineInAllDays: true,
+      keepScrollOffset: true,
       timeSlotColorBuilder: (_, slotStartTime, __, ___) {
         final hour = slotStartTime.hour;
         final isBusinessHours = hour >= 9 && hour < 17;

lib/src/day_view/_internal_day_view_page.dart

@@ -103,8 +103,6 @@ class InternalDayViewPage<T extends Object?> extends StatefulWidget {
   /// Display full day events.
   final FullDayEventBuilder<T> fullDayEventBuilder;
 
-  final ScrollController dayViewScrollController;
-
   /// Flag to display half hours.
   final bool showHalfHours;
 
@@ -120,8 +118,18 @@ class InternalDayViewPage<T extends Object?> extends StatefulWidget {
   /// Settings for half hour indicator lines.
   final HourIndicatorSettings quarterHourIndicatorSettings;
 
-  /// Scroll listener to set every page's last offset
-  final void Function(ScrollController) scrollListener;
+  /// Scroll listener to set every page's last offset.
+  final void Function(
+    int pageIndex,
+    double offset,
+    ZoomScrollController controller,
+  ) scrollListener;
+
+  /// Page index in the parent [PageView].
+  final int pageIndex;
+
+  /// Whether this page is currently visible in parent [PageView].
+  final bool isCurrentPage;
 
   /// Last scroll offset of day view page.
   final double lastScrollOffset;
@@ -176,9 +184,10 @@ class InternalDayViewPage<T extends Object?> extends StatefulWidget {
     required this.minuteSlotSize,
     required this.scrollNotifier,
     required this.fullDayEventBuilder,
-    required this.dayViewScrollController,
     required this.scrollPhysics,
     required this.scrollListener,
+    required this.pageIndex,
+    required this.isCurrentPage,
     required this.dayDetectorBuilder,
     required this.showHalfHours,
     required this.showQuarterHours,
@@ -210,6 +219,17 @@ class _InternalDayViewPageState<T extends Object?>
       initialScrollOffset: widget.lastScrollOffset,
     );
     scrollController.addListener(_scrollControllerListener);
+
+    if (widget.isCurrentPage) {
+      WidgetsBinding.instance.addPostFrameCallback((_) {
+        if (!mounted) return;
+        widget.scrollListener(
+          widget.pageIndex,
+          scrollController.offset,
+          scrollController,
+        );
+      });
+    }
   }
 
   @override
@@ -225,6 +245,23 @@ class _InternalDayViewPageState<T extends Object?>
           (oldWidget.heightPerMinute > 0 ? oldWidget.heightPerMinute : 1.0);
       scrollController.prepareZoomJump(scaledOffset);
     }
+
+    if (!widget.keepScrollOffset &&
+        widget.isCurrentPage &&
+        !oldWidget.isCurrentPage &&
+        scrollController.hasClients) {
+      scrollController.jumpTo(widget.lastScrollOffset);
+    }
+
+    if (widget.isCurrentPage && !oldWidget.isCurrentPage) {
+      widget.scrollListener(
+        widget.pageIndex,
+        scrollController.hasClients
+            ? scrollController.offset
+            : widget.lastScrollOffset,
+        scrollController,
+      );
+    }
   }
 
   @override
@@ -236,7 +273,11 @@ class _InternalDayViewPageState<T extends Object?>
   }
 
   void _scrollControllerListener() {
-    widget.scrollListener(scrollController);
+    widget.scrollListener(
+      widget.pageIndex,
+      scrollController.offset,
+      scrollController,
+    );
   }
 
   /// Builds the background color layer for time slots in the day view.
@@ -325,9 +366,7 @@ class _InternalDayViewPageState<T extends Object?>
                 scrollbars: widget.keepScrollOffset,
               ),
               child: SingleChildScrollView(
-                controller: widget.keepScrollOffset
-                    ? scrollController
-                    : widget.dayViewScrollController,
+                controller: scrollController,
                 physics: widget.scrollPhysics,
                 child: SizedBox(
                   height: widget.height,