Fix the media-send thumbnail row's delete affordance landing on the wrong image mid-reorder.

feature/media-send/src/main/java/org/signal/mediasend/screens/edit/ThumbnailRow.kt

@@ -36,6 +36,7 @@ import androidx.compose.ui.draw.clip
 import androidx.compose.ui.graphics.Color
 import androidx.compose.ui.platform.LocalDensity
 import androidx.compose.ui.platform.LocalInspectionMode
+import androidx.compose.ui.platform.testTag
 import androidx.compose.ui.unit.Dp
 import androidx.compose.ui.unit.dp
 import androidx.core.net.toUri
@@ -53,6 +54,7 @@ import org.signal.core.ui.compose.list.reorderableList
 import org.signal.core.util.ContentTypeUtil
 import org.signal.glide.compose.GlideImage
 import org.signal.mediasend.screens.MediaSendMetrics
+import org.signal.mediasend.test.TestTags
 import kotlin.math.abs
 import kotlin.math.floor
 import kotlin.math.roundToInt
@@ -96,6 +98,15 @@ internal fun ThumbnailRow(
   )
   val isReordering = reorderableListState.draggingItemIndex != null
 
+  // A thumbnail's page has to be looked up by identity rather than taken from its slot in the row: for the length of a
+  // drag the row renders the order that drag has built up while the pager is still on the pre-drag one, so the two
+  // disagree about what any given index refers to. Everything keyed off the pager - the delete affordance and the
+  // fish-eye - has to follow the media rather than the index, or it lands on whatever the drag has shuffled into the
+  // pager's slot and only corrects itself once the new order arrives.
+  val pageIndices = remember(selectedMedia) {
+    selectedMedia.withIndex().associate { (index, media) -> media.uri to index }
+  }
+
   val draggableState = rememberDraggableState { delta ->
     val scaledDelta = delta * (pagerPageSize.toFloat() / itemStride)
     pagerState.dispatchRawDelta(-scaledDelta)
@@ -170,10 +181,12 @@ internal fun ThumbnailRow(
       modifier = if (enabled) Modifier.reorderableList(reorderableListState) else Modifier
     ) {
       itemsIndexed(reorderBuffer.items, key = { _, media -> media.uri }) { index, media ->
-        val padding by remember(index) {
+        val pageIndex = pageIndices[media.uri] ?: index
+
+        val padding by remember(pageIndex) {
           derivedStateOf {
             val currentPosition = pagerState.currentPage + pagerState.currentPageOffsetFraction
-            val distanceFromCenter = abs(index - currentPosition).coerceIn(0f, 1f)
+            val distanceFromCenter = abs(pageIndex - currentPosition).coerceIn(0f, 1f)
             lerp(MAX_PADDING, MIN_PADDING, distanceFromCenter)
           }
         }
@@ -184,13 +197,14 @@ internal fun ThumbnailRow(
           modifier = Modifier.clip(MediaSendMetrics.SelectedMediaPreviewShape)
         ) {
           DeleteBox(
-            enabled = pagerState.currentPage == index
+            enabled = pagerState.currentPage == pageIndex,
+            testTag = TestTags.thumbnailRowDeleteIcon(media.uri.toString())
           ) {
             Thumbnail(
               media = media,
               modifier = Modifier
                 .padding(horizontal = padding)
-                .clickable(enabled = enabled) { onThumbnailClick(index) }
+                .clickable(enabled = enabled) { onThumbnailClick(pageIndex) }
             )
           }
         }
@@ -206,6 +220,7 @@ private fun lerp(start: Dp, stop: Dp, fraction: Float): Dp {
 @Composable
 private fun DeleteBox(
   enabled: Boolean,
+  testTag: String? = null,
   content: @Composable () -> Unit
 ) {
   Box {
@@ -221,6 +236,7 @@ private fun DeleteBox(
           .size(MediaSendMetrics.SelectedMediaPreviewSize)
           .padding(10.dp)
           .align(Alignment.Center)
+          .then(if (testTag != null) Modifier.testTag(testTag) else Modifier)
       )
     }
   }

feature/media-send/src/main/java/org/signal/mediasend/test/TestTags.kt

@@ -19,6 +19,11 @@ object TestTags {
   const val MEDIA_EDITOR_TOOLBAR_ADD_MEDIA_BUTTON = "media_editor_toolbar_add_media_button"
   const val MEDIA_EDITOR_TOOLBAR_MUTE_BUTTON = "media_editor_toolbar_mute_button"
 
+  // Media Edit Screen
+
+  /** Tag for the delete affordance the thumbnail row puts on the media at [uri]. */
+  fun thumbnailRowDeleteIcon(uri: String): String = "thumbnail_row_delete_icon_$uri"
+
   // Media Capture Screen
   const val MEDIA_CAPTURE_SCREEN = "media_capture_screen"
   const val MEDIA_CAPTURE_CAMERA_TOGGLE = "media_capture_camera_toggle"

feature/media-send/src/test/java/org/signal/mediasend/screens/edit/ThumbnailRowTest.kt

@@ -0,0 +1,151 @@
+/*
+ * Copyright 2026 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.signal.mediasend.screens.edit
+
+import android.app.Application
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.size
+import androidx.compose.foundation.pager.rememberPagerState
+import androidx.compose.runtime.CompositionLocalProvider
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.geometry.Offset
+import androidx.compose.ui.platform.LocalInspectionMode
+import androidx.compose.ui.platform.testTag
+import androidx.compose.ui.test.hasClickAction
+import androidx.compose.ui.test.junit4.createComposeRule
+import androidx.compose.ui.test.onNodeWithTag
+import androidx.compose.ui.test.performTouchInput
+import androidx.compose.ui.unit.dp
+import androidx.core.net.toUri
+import androidx.test.core.app.ApplicationProvider
+import org.junit.Assert.assertEquals
+import org.junit.Rule
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.annotation.Config
+import org.signal.core.models.media.Media
+import org.signal.core.ui.CoreUiDependenciesRule
+import org.signal.core.ui.compose.theme.SignalTheme
+import org.signal.mediasend.test.TestTags
+
+/**
+ * Covers the delete affordance staying on the media the pager is showing for the length of a reorder. The row renders
+ * the order the drag has built up while the pager stays on the pre-drag one until the drop lands in state, so whether
+ * the two still agree about which thumbnail is the focused one is not something either of them can be asked alone.
+ */
+@RunWith(RobolectricTestRunner::class)
+@Config(application = Application::class, qualifiers = "w400dp-h800dp")
+class ThumbnailRowTest {
+
+  @get:Rule
+  val composeTestRule = createComposeRule()
+
+  @get:Rule
+  val coreUiDependenciesRule = CoreUiDependenciesRule(ApplicationProvider.getApplicationContext())
+
+  private val reorders = mutableListOf<Pair<Int, Int>>()
+
+  @Test
+  fun `Given the focused thumbnail is dragged past its neighbour, when it has not been dropped, then the delete affordance is still on it`() {
+    setContent()
+
+    composeTestRule.onNodeWithTag(HOST).performTouchInput {
+      down(thumbnailCenter(0))
+      advanceEventTime(LONG_PRESS_MS)
+      moveTo(thumbnailCenter(1) + Offset(overshootPx, 0f))
+    }
+    composeTestRule.waitForIdle()
+
+    composeTestRule.onNodeWithTag(deleteIconTagFor(0)).assertExists()
+    composeTestRule.onNodeWithTag(deleteIconTagFor(1)).assertDoesNotExist()
+  }
+
+  /** Guards the test above from passing on a drag that never swapped anything. */
+  @Test
+  fun `Given the focused thumbnail is dragged past its neighbour, when it is dropped, then the two have traded places`() {
+    setContent()
+
+    composeTestRule.onNodeWithTag(HOST).performTouchInput {
+      down(thumbnailCenter(0))
+      advanceEventTime(LONG_PRESS_MS)
+      moveTo(thumbnailCenter(1) + Offset(overshootPx, 0f))
+      up()
+    }
+    composeTestRule.waitForIdle()
+
+    assertEquals(listOf(0 to 1), reorders)
+  }
+
+  /**
+   * Read off the laid-out thumbnails rather than derived from their size, so the gesture lands where they actually are
+   * once the row's fish-eye padding has had its say.
+   */
+  private fun thumbnailCenter(index: Int): Offset {
+    val thumbnail = composeTestRule.onAllNodes(hasClickAction())[index].fetchSemanticsNode()
+
+    return Offset(
+      thumbnail.positionInRoot.x + thumbnail.size.width / 2f,
+      thumbnail.positionInRoot.y + thumbnail.size.height / 2f
+    )
+  }
+
+  /** Enough past the neighbour's center to be clear of the midpoint a swap is decided on. */
+  private val overshootPx: Float
+    get() = with(composeTestRule.density) { OVERSHOOT.toPx() }
+
+  private fun deleteIconTagFor(index: Int): String = TestTags.thumbnailRowDeleteIcon(MEDIA[index].uri.toString())
+
+  /**
+   * Inspected rather than loaded: a thumbnail emits nothing at all until Glide has something to put in it, and there is
+   * nothing to load here, so the row would otherwise lay out as a strip of empty slots with no geometry for a drag to
+   * work against.
+   */
+  private fun setContent() {
+    composeTestRule.setContent {
+      SignalTheme {
+        CompositionLocalProvider(LocalInspectionMode provides true) {
+          Box(modifier = Modifier.size(ROW_WIDTH.dp, ROW_HEIGHT.dp).testTag(HOST)) {
+            ThumbnailRow(
+              selectedMedia = MEDIA,
+              pagerState = rememberPagerState(initialPage = 0, pageCount = { MEDIA.size }),
+              onReorder = { fromIndex, toIndex -> reorders += fromIndex to toIndex }
+            )
+          }
+        }
+      }
+    }
+
+    composeTestRule.waitForIdle()
+  }
+
+  private companion object {
+    private const val HOST = "thumbnail_row_host"
+    private const val LONG_PRESS_MS = 600L
+    private const val ROW_WIDTH = 400f
+    private const val ROW_HEIGHT = 100f
+
+    private val OVERSHOOT = 10.dp
+
+    private val MEDIA: List<Media> = (0 until 3).map { index ->
+      Media(
+        uri = "content://media/$index".toUri(),
+        contentType = "image/jpeg",
+        date = index.toLong(),
+        width = 100,
+        height = 100,
+        size = 1024,
+        duration = 0,
+        isBorderless = false,
+        isVideoGif = false,
+        bucketId = "bucket",
+        caption = null,
+        transformProperties = null,
+        fileName = "media_$index.jpg"
+      )
+    }
+  }
+}