Remove trailing space from backup key transform.

app/src/main/java/org/thoughtcrime/securesms/registrationv3/ui/restore/BackupKeyVisualTransformation.kt

@@ -25,15 +25,23 @@ class BackupKeyVisualTransformation(private val chunkSize: Int) : VisualTransfor
       }
     }
 
+    val transformed = output.trimEnd()
+
     return TransformedText(
-      text = AnnotatedString(output),
-      offsetMapping = BackupKeyVisualTransformation(chunkSize)
+      text = AnnotatedString(transformed),
+      offsetMapping = BackupKeyVisualTransformation(chunkSize, text.length)
     )
   }
 
-  private class BackupKeyVisualTransformation(private val chunkSize: Int) : OffsetMapping {
+  private class BackupKeyVisualTransformation(private val chunkSize: Int, private val inputSize: Int) : OffsetMapping {
     override fun originalToTransformed(offset: Int): Int {
-      return offset + (offset / chunkSize)
+      val transformed = offset + (offset / chunkSize)
+
+      return when {
+        inputSize == 0 -> 0
+        offset == inputSize && offset >= chunkSize && offset % chunkSize == 0 -> transformed - 1
+        else -> transformed
+      }
     }
 
     override fun transformedToOriginal(offset: Int): Int {

app/src/test/java/org/thoughtcrime/securesms/registrationv3/ui/restore/BackupKeyVisualTransformationTest.kt

@@ -0,0 +1,95 @@
+/*
+ * Copyright 2025 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.thoughtcrime.securesms.registrationv3.ui.restore
+
+import androidx.compose.ui.text.AnnotatedString
+import assertk.assertThat
+import assertk.assertions.isEqualTo
+import org.junit.Test
+
+class BackupKeyVisualTransformationTest {
+
+  companion object {
+    private const val TEST_KEY = "AAAABBBBCCCCDDDDEEEEFFFFGGGGHHHHIIIIJJJJKKKKLLLLMMMMNNNNOOOOPPPP"
+    private const val FORMATTED_TEST_KEY = "AAAA BBBB CCCC DDDD EEEE FFFF GGGG HHHH IIII JJJJ KKKK LLLL MMMM NNNN OOOO PPPP"
+  }
+
+  @Test
+  fun `Given a blank entry, ensure offset mapping is 0`() {
+    val testSubject = BackupKeyVisualTransformation(chunkSize = 4)
+
+    val result = testSubject.filter(AnnotatedString(""))
+
+    assertThat(result.offsetMapping.originalToTransformed(0)).isEqualTo(0)
+  }
+
+  @Test
+  fun `Given a single entry, ensure offset mapping is 0`() {
+    val testSubject = BackupKeyVisualTransformation(chunkSize = 4)
+
+    val result = testSubject.filter(AnnotatedString("A"))
+
+    assertThat(result.offsetMapping.originalToTransformed(1)).isEqualTo(1)
+  }
+
+  @Test
+  fun `Given a backup key, ensure a space comes after every 4 characters`() {
+    val testSubject = BackupKeyVisualTransformation(chunkSize = 4)
+    val text = AnnotatedString(TEST_KEY)
+    val expected = AnnotatedString(FORMATTED_TEST_KEY)
+
+    val transformed = testSubject.filter(text)
+    val result = transformed.text
+
+    assertThat(result).isEqualTo(expected)
+  }
+
+  @Test
+  fun `Given output length, when I originalToTransformed, then I expect proper output`() {
+    val testSubject = BackupKeyVisualTransformation(chunkSize = 4)
+    val text = AnnotatedString("AAAAAAAA")
+
+    val transformed = testSubject.filter(text)
+    val result = transformed.offsetMapping
+
+    assertThat(result.originalToTransformed(text.length)).isEqualTo(9)
+  }
+
+  @Test
+  fun `When I originalToTransformed, then I expect proper output`() {
+    val testSubject = BackupKeyVisualTransformation(chunkSize = 4)
+    val text = AnnotatedString("AAAAA")
+    val expected = listOf(0, 1, 2, 3, 5, 6)
+
+    val transformed = testSubject.filter(text)
+    val result = transformed.offsetMapping
+    val mapping = (0..text.length).map { result.originalToTransformed(it) }
+    assertThat(mapping).isEqualTo(expected)
+  }
+
+  @Test
+  fun `Given output length, when I transformedToOriginal, then I expect proper output`() {
+    val testSubject = BackupKeyVisualTransformation(chunkSize = 4)
+    val text = AnnotatedString("AAAAAAAA")
+
+    val transformed = testSubject.filter(text)
+    val result = transformed
+
+    assertThat(result.offsetMapping.transformedToOriginal(result.text.length)).isEqualTo(text.length)
+  }
+
+  @Test
+  fun `When I transformedToOriginal, then I expect proper output`() {
+    val testSubject = BackupKeyVisualTransformation(chunkSize = 4)
+    val original = AnnotatedString(TEST_KEY)
+    val expected = (0 until original.length).toSet()
+
+    val transformed = testSubject.filter(original)
+    val result = transformed.offsetMapping
+    val mapping = transformed.text.indices.map { result.transformedToOriginal(it) }
+    assertThat(mapping.toSet()).isEqualTo(expected)
+  }
+}