From bf90ad0b6a8ff72e61f55fd72660f4c135a48d94 Mon Sep 17 00:00:00 2001
From: Thibault Deckers <thibault.deckers@gmail.com>
Date: Mon, 4 May 2020 13:02:53 +0900
Subject: [PATCH] improved scroll accuracy after scaling

---
 lib/widgets/album/grid/scaling.dart         | 9 +++++++--
 lib/widgets/album/thumbnail_collection.dart | 1 +
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/lib/widgets/album/grid/scaling.dart b/lib/widgets/album/grid/scaling.dart
index 183b8c7a5..3169b57c7 100644
--- a/lib/widgets/album/grid/scaling.dart
+++ b/lib/widgets/album/grid/scaling.dart
@@ -13,6 +13,7 @@ import 'package:provider/provider.dart';
 
 class GridScaleGestureDetector extends StatefulWidget {
   final GlobalKey scrollableKey;
+  final ValueNotifier<double> appBarHeightNotifier;
   final ValueNotifier<double> extentNotifier;
   final Size mqSize;
   final double mqHorizontalPadding;
@@ -20,6 +21,7 @@ class GridScaleGestureDetector extends StatefulWidget {
 
   const GridScaleGestureDetector({
     this.scrollableKey,
+    @required this.appBarHeightNotifier,
     @required this.extentNotifier,
     @required this.mqSize,
     @required this.mqHorizontalPadding,
@@ -120,8 +122,11 @@ class _GridScaleGestureDetectorState extends State<GridScaleGestureDetector> {
             final scrollableContext = widget.scrollableKey.currentContext;
             final gridSize = (scrollableContext.findRenderObject() as RenderBox).size;
             final sectionLayout = Provider.of<SectionedListLayout>(context, listen: false);
-            final tileRect = sectionLayout.getTileRect(_metadata.entry);
-            final scrollOffset = (tileRect?.top ?? 0) - gridSize.height / 2;
+            final tileRect = sectionLayout.getTileRect(_metadata.entry) ?? Rect.zero;
+            // most of the time the app bar will be scrolled away after scaling,
+            // so we compensate for it to center the focal point thumbnail
+            final appBarHeight = widget.appBarHeightNotifier.value;
+            final scrollOffset = tileRect.top + (tileRect.height - gridSize.height) / 2 + appBarHeight;
             viewportClosure.offset.jumpTo(max(.0, scrollOffset));
             _applyingScale = false;
           });
diff --git a/lib/widgets/album/thumbnail_collection.dart b/lib/widgets/album/thumbnail_collection.dart
index ebbefa2cc..901039601 100644
--- a/lib/widgets/album/thumbnail_collection.dart
+++ b/lib/widgets/album/thumbnail_collection.dart
@@ -38,6 +38,7 @@ class ThumbnailCollection extends StatelessWidget {
               final draggable = _buildDraggableScrollView(scrollView);
               final scaler = GridScaleGestureDetector(
                 scrollableKey: _scrollableKey,
+                appBarHeightNotifier: _appBarHeightNotifier,
                 extentNotifier: _tileExtentNotifier,
                 mqSize: mqSize,
                 mqHorizontalPadding: mqHorizontalPadding,