Add a smaller test for maintaining this in the long-term.

Author: ntwigg

jvm/selfie-lib/src/commonMain/kotlin/com/diffplug/selfie/SelfieImplementations.kt

@@ -212,7 +212,7 @@ class StringSelfie(
  * Returns a serialized form of only the given facets if they are available, silently omits missing
  * facets.
  */
-private fun serializeOnlyFacets(snapshot: Snapshot, keys: Collection<String>): String {
+internal fun serializeOnlyFacets(snapshot: Snapshot, keys: Collection<String>): String {
   val writer = StringBuilder()
   for (key in keys) {
     if (key.isEmpty()) {

jvm/selfie-lib/src/commonTest/kotlin/com/diffplug/selfie/SelfieImplementationsTest.kt

@@ -0,0 +1,33 @@
+/*
+ * Copyright (C) 2025 DiffPlug
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.diffplug.selfie
+
+import io.kotest.assertions.throwables.shouldThrow
+import io.kotest.matchers.shouldBe
+import kotlin.test.Test
+
+class SelfieImplementationsTest {
+  @Test
+  fun issue_529() {
+    val empty = Snapshot.of("")
+    val emptyPlusFacet = Snapshot.of("").plusFacet("new-facet", "new-facet-value")
+    val exception =
+        shouldThrow<StringIndexOutOfBoundsException> {
+          serializeOnlyFacets(empty, listOf("new-facet"))
+        }
+    exception.message shouldBe "String index out of range: -1"
+  }
+}