Preserve scroll position in AssetBrowser refresh

Author: trende2001

game/addons/tools/Code/Editor/AssetBrowser/AssetBrowser.cs

@@ -130,6 +130,104 @@ public AssetListViewMode ViewModeType
 
 	private FileSystemWatcher watcher;
 
+	int lastKnownScrollPosition = 0;
+
+	private struct ScrollAnchor
+	{
+		public string Key;
+		public int Index;
+		public int Scroll;
+
+		public ScrollAnchor( string key, int index, int scroll )
+		{
+			Key = key;
+			Index = index;
+			Scroll = scroll;
+		}
+	}
+
+	private bool TryGetItemKey( object item, out string key )
+	{
+		key = item switch
+		{
+			AssetEntry ae => ae.FileInfo.FullName ?? ae.Asset?.AbsolutePath,
+			DirectoryEntry de => de.DirectoryInfo.FullName,
+			_ => null
+		};
+
+		return !string.IsNullOrEmpty( key );
+	}
+
+	private ScrollAnchor CaptureScrollAnchor()
+	{
+		if ( AssetList?.VerticalScrollbar?.IsValid() == true )
+		{
+			lastKnownScrollPosition = AssetList.VerticalScrollbar.Value;
+		}
+
+		if ( AssetList?.Items.Any() != true )
+			return new ScrollAnchor( null, 0, lastKnownScrollPosition );
+
+		var itemHeight = MathF.Max( AssetList.ItemSize.y + AssetList.ItemSpacing.y, 1f );
+		int index = (int)MathF.Floor( lastKnownScrollPosition / itemHeight );
+		index = Math.Clamp( index, 0, AssetList.Items.Count() - 1 );
+
+		string key = null;
+		var item = AssetList.Items.ElementAt( index );
+		TryGetItemKey( item, out key );
+
+		return new ScrollAnchor( key, index, lastKnownScrollPosition );
+	}
+
+	private void RestoreScrollPosition( int target )
+	{
+		if ( AssetList?.VerticalScrollbar?.IsValid() != true )
+			return;
+
+		void Apply()
+		{
+			if ( AssetList?.VerticalScrollbar?.IsValid() != true )
+				return;
+
+			var scrollbar = AssetList.VerticalScrollbar;
+			var clamped = Math.Clamp( target, scrollbar.Minimum, scrollbar.Maximum );
+			scrollbar.Value = clamped;
+			AssetList.SmoothScrollTarget = 0;
+			lastKnownScrollPosition = clamped;
+		}
+
+		Apply();
+		MainThread.Queue( Apply );
+	}
+
+	private void RestoreScrollAnchor( ScrollAnchor anchor )
+	{
+		if ( AssetList?.VerticalScrollbar?.IsValid() != true )
+			return;
+
+		var desired = anchor.Scroll;
+
+		if ( !string.IsNullOrEmpty( anchor.Key ) && AssetList.Items.Any() )
+		{
+			var match = AssetList.Items.Select( ( x, i ) => (x, i) )
+				.FirstOrDefault( tuple => TryGetItemKey( tuple.x, out var key ) && StringComparer.OrdinalIgnoreCase.Equals( key, anchor.Key ) );
+
+			if ( match != default && match.i >= 0 )
+			{
+				var itemHeight = MathF.Max( AssetList.ItemSize.y + AssetList.ItemSpacing.y, 1f );
+				desired = (int)(match.i * itemHeight);
+			}
+		}
+		else if ( AssetList.Items.Any() )
+		{
+			var idx = Math.Clamp( anchor.Index, 0, AssetList.Items.Count() - 1 );
+			var itemHeight = MathF.Max( AssetList.ItemSize.y + AssetList.ItemSpacing.y, 1f );
+			desired = (int)(idx * itemHeight);
+		}
+
+		RestoreScrollPosition( desired );
+	}
+
 	public AssetBrowser( Widget parent ) : this( parent, null )
 	{
 
@@ -410,7 +508,7 @@ private async Task<bool> UpdateAssetListAsync( bool recursive, CancellationToken
 		List<object> items = new List<object>();
 		var tagCounts = new Dictionary<string, int>();
 
-		AssetList.Clear();
+		var anchor = CaptureScrollAnchor();
 
 		await Task.Run( () =>
 		{
@@ -501,6 +599,9 @@ await Task.Run( () =>
 			return false;
 
 		AssetList.SetItems( items );
+
+		RestoreScrollAnchor( anchor );
+
 		if ( !string.IsNullOrEmpty( lastSortColumn ) )
 		{
 			SortAssetList( lastSortColumn, lastSortAscending );
@@ -522,6 +623,7 @@ await Task.Run( () =>
 
 	private void SortAssetList( string sortBy, bool ascending )
 	{
+		var anchor = CaptureScrollAnchor();
 		List<object> items;
 
 		switch ( sortBy )
@@ -594,6 +696,8 @@ private void SortAssetList( string sortBy, bool ascending )
 			items.Reverse();
 		}
 		AssetList.SetItems( items );
+		RestoreScrollAnchor( anchor );
+		MainThread.Queue( () => RestoreScrollAnchor( anchor ) );
 
 		lastSortColumn = sortBy;
 		lastSortAscending = ascending;