Improvments from upstream + own fixes

Directory.Build.props

@@ -10,12 +10,14 @@
     <MSBuildEnableWorkloadResolver>false</MSBuildEnableWorkloadResolver>
   </PropertyGroup>
 
-  <PropertyGroup Condition="'$(TargetFramework)' == 'net8.0'">
+  <!-- 'NET6_0_OR_GREATER' equivalent -->
+  <PropertyGroup Condition="'$(TargetFrameworkIdentifier)' == '.NETCoreApp' AND $([MSBuild]::VersionGreaterThanOrEquals($(TargetFrameworkVersion), '6.0'))">
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
     <WarningsAsErrors>nullable</WarningsAsErrors>
   </PropertyGroup>
 
-  <PropertyGroup Condition="'$(TargetFramework)' != 'net8.0'">
+  <!-- '!NET6_0_OR_GREATER' equivalent -->
+  <PropertyGroup Condition="!('$(TargetFrameworkIdentifier)' == '.NETCoreApp' AND $([MSBuild]::VersionGreaterThanOrEquals($(TargetFrameworkVersion), '6.0')))">
     <NoWarn>$(NoWarn);8604;8602</NoWarn>
   </PropertyGroup>
 

src/Avalonia.Controls.TreeDataGrid/Experimental/Data/Core/LightweightObservableBase.cs

@@ -1,4 +1,5 @@
-using System;
+using System;
+using System.Buffers;
 using System.Collections.Generic;
 using System.Threading;
 
@@ -19,7 +20,7 @@ public abstract class LightweightObservableBase<T> : IObservable<T>
         private List<IObserver<T>>? _observers = [];
 
         public bool HasObservers => _observers?.Count > 0;
-        
+
         public IDisposable Subscribe(IObserver<T> observer)
         {
             _ = observer ?? throw new ArgumentNullException(nameof(observer));
@@ -117,32 +118,61 @@ protected void PublishNext(T value)
             if (Volatile.Read(ref _observers) != null)
             {
                 IObserver<T>[]? observers = null;
-                IObserver<T>? singleObserver = null;
+                int count = 0;
+
+                // Optimize for the common case of 1/2/3 observers.
+                IObserver<T>? observer0 = null;
+                IObserver<T>? observer1 = null;
+                IObserver<T>? observer2 = null;
                 lock (this)
                 {
                     if (_observers == null)
                     {
                         return;
                     }
-                    if (_observers.Count == 1)
-                    {
-                        singleObserver = _observers[0];
-                    }
-                    else
+
+                    count = _observers.Count;
+                    switch (count)
                     {
-                        observers = _observers.ToArray();
+                        case 3:
+                            observer0 = _observers[0];
+                            observer1 = _observers[1];
+                            observer2 = _observers[2];
+                            break;
+                        case 2:
+                            observer0 = _observers[0];
+                            observer1 = _observers[1];
+                            break;
+                        case 1:
+                            observer0 = _observers[0];
+                            break;
+                        case 0:
+                            return;
+                        default:
+                            {
+                                observers = ArrayPool<IObserver<T>>.Shared.Rent(count);
+                                _observers.CopyTo(observers);
+                                break;
+                            }
                     }
                 }
-                if (singleObserver != null)
+
+                if (observer0 != null)
                 {
-                    singleObserver.OnNext(value);
+                    observer0.OnNext(value);
+                    observer1?.OnNext(value);
+                    observer2?.OnNext(value);
                 }
-                else
+                else if (observers != null)
                 {
-                    foreach (var observer in observers!)
+                    for (int i = 0; i < count; i++)
                     {
-                        observer.OnNext(value);
+                        observers[i].OnNext(value);
+                        // Avoid memory leak by clearing the reference.
+                        observers[i] = null!;
                     }
+
+                    ArrayPool<IObserver<T>>.Shared.Return(observers);
                 }
             }
         }

src/Avalonia.Controls.TreeDataGrid/Primitives/TreeDataGridPresenterBase.cs

@@ -739,8 +739,8 @@ private void RecycleElementOnItemRemoved(Control element)
                 _focusedIndex = -1;
             }
 
-            UnrealizeElementOnItemRemoved(element);
             element.IsVisible = false;
+            UnrealizeElementOnItemRemoved(element);
             ElementFactory!.RecycleElement(element);
             _scrollViewer?.UnregisterAnchorCandidate(element);
         }

src/Avalonia.Controls.TreeDataGrid/Primitives/TreeDataGridTextCell.cs

@@ -1,6 +1,4 @@
 using System.ComponentModel;
-using System.Globalization;
-using System.Reflection;
 using Avalonia.Controls.Models.TreeDataGrid;
 using Avalonia.Controls.Selection;
 using Avalonia.Media;