fix issues with subfolders

src/SMAPI.Tests/Core/AssetNameTests.cs

@@ -243,6 +243,20 @@ namespace SMAPI.Tests.Core
             return result;
         }
 
+        [TestCase("Mods/SomeMod/SomeSubdirectory", "Mods/Some", true, ExpectedResult = true)]
+        [TestCase("Mods/SomeMod/SomeSubdirectory", "Mods/Some", false, ExpectedResult = false)]
+        public bool StartsWith_SubfolderWithPartial(string mainAssetName, string otherAssetName, bool allowSubfolder)
+        {
+            // arrange
+            mainAssetName = PathUtilities.NormalizeAssetName(mainAssetName);
+
+            // act
+            AssetName name = AssetName.Parse(mainAssetName, _ => null);
+
+            // assert value
+            return name.StartsWith(otherAssetName, allowPartialWord: true, allowSubfolder: allowSubfolder);
+        }
+
 
         /****
         ** GetHashCode

src/SMAPI.Tests/SMAPI.Tests.csproj

@@ -19,6 +19,7 @@
     <PackageReference Include="Moq" Version="4.18.1" />
     <PackageReference Include="Newtonsoft.Json" Version="13.0.1" />
     <PackageReference Include="NUnit" Version="3.13.3" />
+    <PackageReference Include="NUnit3TestAdapter" Version="4.2.1" PrivateAssets="all" IncludeAssets="runtime; build; native; contentfiles; analyzers; buildtransitive" />
   </ItemGroup>
 
   <ItemGroup>

src/SMAPI/Framework/Content/AssetName.cs

@@ -162,8 +162,8 @@ namespace StardewModdingAPI.Framework.Content
                     if (prefixHasMore)
                         return false;
 
-                    // possible match: all prefix segments matched
+                    // possible match: all prefix segments matched.
-                    return allowSubfolder || !curParts.Remainder.Contains(pathSeparators, StringComparison.Ordinal);
+                    return allowSubfolder || (pathSeparators.Contains(trimmed[^1]) ? curParts.Remainder.Length == 0 : curParts.Current.Length == 0);
                 }
 
                 // match: previous segments matched exactly and both reached the end
@@ -192,7 +192,7 @@ namespace StardewModdingAPI.Framework.Content
                         return false;
 
                     // possible match
-                    return allowSubfolder || !curParts.Remainder.Contains(pathSeparators, StringComparison.Ordinal);
+                    return allowSubfolder || (pathSeparators.Contains(trimmed[^1]) ? curParts.Remainder.IndexOfAny(ToolkitPathUtilities.PossiblePathSeparators) < 0 : curParts.Remainder.Length == 0);
                 }
             }
         }
@@ -203,7 +203,7 @@ namespace StardewModdingAPI.Framework.Content
             if (assetFolder is null)
                 return false;
 
-            return this.StartsWith(assetFolder + "/", allowPartialWord: false, allowSubfolder: false);
+            return this.StartsWith(assetFolder + ToolkitPathUtilities.PreferredPathSeparator, allowPartialWord: false, allowSubfolder: false);
         }
 
         /// <inheritdoc />