From 87e2ed810794d6d3ea72cb2b75d744adb278b0ab Mon Sep 17 00:00:00 2001
From: Tom Edwards <t.edwards@enscape3d.com>
Date: Sat, 1 Apr 2023 13:00:52 +0200
Subject: [PATCH] Don't detach from ScrollViewer when detached from visual tree
 Fixes offset resetting when switching between tabs

---
 .../Presenters/ScrollContentPresenter.cs      | 27 ++++++++---------
 src/Avalonia.Controls/Primitives/ScrollBar.cs | 29 +++++++++++--------
 src/Avalonia.Controls/ScrollViewer.cs         |  7 +++--
 3 files changed, 36 insertions(+), 27 deletions(-)

diff --git a/src/Avalonia.Controls/Presenters/ScrollContentPresenter.cs b/src/Avalonia.Controls/Presenters/ScrollContentPresenter.cs
index 6552ee6dbd..0efa8d38f8 100644
--- a/src/Avalonia.Controls/Presenters/ScrollContentPresenter.cs
+++ b/src/Avalonia.Controls/Presenters/ScrollContentPresenter.cs
@@ -304,15 +304,23 @@ namespace Avalonia.Controls.Presenters
         /// </remarks>
         protected internal virtual void AttachToScrollViewer()
         {
-            _ownerSubscriptions?.Dispose();
-
-            var owner = _owner = this.FindAncestorOfType<ScrollViewer>();
+            var owner = this.FindAncestorOfType<ScrollViewer>();
 
             if (owner == null)
             {
+                _owner = null;
+                _ownerSubscriptions?.Dispose();
+                _ownerSubscriptions = null;
                 return;
             }
 
+            if (owner == _owner)
+            {
+                return;
+            }
+
+            _ownerSubscriptions?.Dispose();
+
             var subscriptionDisposables = new IDisposable?[]
             {
                 IfUnset(CanHorizontallyScrollProperty, p => Bind(p, owner.GetObservable(ScrollViewer.HorizontalScrollBarVisibilityProperty, NotDisabled), Data.BindingPriority.Template)),
@@ -322,19 +330,12 @@ namespace Avalonia.Controls.Presenters
                 IfUnset(ContentProperty, p => Bind(p, owner.GetBindingObservable(ContentProperty), Data.BindingPriority.Template)),
             }.Where(d => d != null).Cast<IDisposable>().ToArray();
 
+            _owner = owner;
             _ownerSubscriptions = new CompositeDisposable(subscriptionDisposables);
 
             static bool NotDisabled(ScrollBarVisibility v) => v != ScrollBarVisibility.Disabled;
 
-            IDisposable? IfUnset<T>(T property, Func<T, IDisposable> func) where T : AvaloniaProperty => GetValueStore().IsSet(property) ? null : func(property);
-        }
-
-
-        protected override void OnDetachedFromVisualTree(VisualTreeAttachmentEventArgs e)
-        {
-            _ownerSubscriptions?.Dispose();
-            _owner = null;
-            base.OnDetachedFromVisualTree(e);
+            IDisposable? IfUnset<T>(T property, Func<T, IDisposable> func) where T : AvaloniaProperty => IsSet(property) ? null : func(property);
         }
 
         /// <inheritdoc/>
@@ -708,7 +709,7 @@ namespace Avalonia.Controls.Presenters
                 }
                 CoerceValue(OffsetProperty);
             }
-            else if (change.Property == ViewportProperty && _owner != null)
+            else if (change.Property == ViewportProperty)
             {
                 if (_owner != null)
                 {
diff --git a/src/Avalonia.Controls/Primitives/ScrollBar.cs b/src/Avalonia.Controls/Primitives/ScrollBar.cs
index 29775f4526..37aa1ebffd 100644
--- a/src/Avalonia.Controls/Primitives/ScrollBar.cs
+++ b/src/Avalonia.Controls/Primitives/ScrollBar.cs
@@ -202,15 +202,23 @@ namespace Avalonia.Controls.Primitives
         /// </remarks>
         protected internal virtual void AttachToScrollViewer()
         {
-            _ownerSubscriptions?.Dispose();
-
-            var owner = _owner = this.FindAncestorOfType<ScrollViewer>();
+            var owner = this.FindAncestorOfType<ScrollViewer>();
 
             if (owner == null)
             {
+                _owner = null;
+                _ownerSubscriptions?.Dispose();
+                _ownerSubscriptions = null;
                 return;
             }
 
+            if (owner == _owner)
+            {
+                return;
+            }
+
+            _ownerSubscriptions?.Dispose();
+
             var visibilitySource = Orientation == Orientation.Horizontal ? ScrollViewer.HorizontalScrollBarVisibilityProperty : ScrollViewer.VerticalScrollBarVisibilityProperty;
 
             var subscriptionDisposables = new IDisposable?[]
@@ -224,21 +232,15 @@ namespace Avalonia.Controls.Primitives
                 IfUnset(SmallChangeProperty, p => Bind(p, owner.GetObservable(ScrollViewer.SmallChangeProperty).Select(ExtractOrdinate), BindingPriority.Template))
             }.Where(d => d != null).Cast<IDisposable>().ToArray();
 
+            _owner = owner;
             _ownerSubscriptions = new CompositeDisposable(subscriptionDisposables);
 
-            IDisposable? IfUnset<T>(T property, Func<T, IDisposable> func) where T : AvaloniaProperty => GetValueStore().IsSet(property) ? null : func(property);
+            IDisposable? IfUnset<T>(T property, Func<T, IDisposable> func) where T : AvaloniaProperty => IsSet(property) ? null : func(property);
         }
 
         private double ExtractOrdinate(Vector v) => Orientation == Orientation.Horizontal ? v.X : v.Y;
         private double ExtractOrdinate(Size v) => Orientation == Orientation.Horizontal ? v.Width : v.Height;
 
-        protected override void OnDetachedFromVisualTree(VisualTreeAttachmentEventArgs e)
-        {
-            _ownerSubscriptions?.Dispose();
-            _owner = null;
-            base.OnDetachedFromVisualTree(e);
-        }
-
         protected override void OnKeyDown(KeyEventArgs e)
         {
             if (e.Key == Key.PageUp)
@@ -260,7 +262,10 @@ namespace Avalonia.Controls.Primitives
             if (change.Property == OrientationProperty)
             {
                 UpdatePseudoClasses(change.GetNewValue<Orientation>());
-                AttachToScrollViewer(); // there's no way to manually refresh bindings, so reapply them
+                if (IsAttachedToVisualTree)
+                {
+                    AttachToScrollViewer(); // there's no way to manually refresh bindings, so reapply them
+                }
             }
             else if (change.Property == AllowAutoHideProperty)
             {
diff --git a/src/Avalonia.Controls/ScrollViewer.cs b/src/Avalonia.Controls/ScrollViewer.cs
index 26b2a030c5..a7188c6226 100644
--- a/src/Avalonia.Controls/ScrollViewer.cs
+++ b/src/Avalonia.Controls/ScrollViewer.cs
@@ -660,8 +660,11 @@ namespace Avalonia.Controls
         private void CalculatedPropertiesChanged()
         {
             var newMaximum = ScrollBarMaximum;
-            RaisePropertyChanged(ScrollBarMaximumProperty, _oldMaximum, newMaximum);
-            _oldMaximum = newMaximum;
+            if (newMaximum != _oldMaximum)
+            {
+                RaisePropertyChanged(ScrollBarMaximumProperty, _oldMaximum, newMaximum);
+                _oldMaximum = newMaximum;
+            }
 
             if (_logicalScrollable?.IsLogicalScrollEnabled == true)
             {