fix(ui): prevent animation flicker when backfilling message history

- Remove AnimatedVisibility wrapper from navigation bar and replace with conditional if statement
- Add comment explaining why animations are deliberately avoided on screen swaps to prevent layout thrashing
- Update MessageArrivalTracker.arrivals() to filter out backfilled messages from arrival detection
- Add lastKnownIndex tracking to distinguish between backfilled history and new messages
- Update filter logic to only report messages newer than the last known message as arrivals
- Add comprehensive test coverage for backfill scenarios: messages loaded behind latest and new arrivals alongside backfill
- Prevents transcript flicker on short conversations by not animating backfilled history rows

app/src/main/java/com/bitchat/android/ui/MainScaffold.kt

@@ -1,11 +1,6 @@
 package com.bitchat.android.ui
 
 import androidx.activity.compose.BackHandler
-import androidx.compose.animation.AnimatedVisibility
-import androidx.compose.animation.fadeIn
-import androidx.compose.animation.fadeOut
-import androidx.compose.animation.slideInVertically
-import androidx.compose.animation.slideOutVertically
 import androidx.compose.foundation.background
 import androidx.compose.foundation.layout.Arrangement
 import androidx.compose.foundation.layout.Box
@@ -105,11 +100,11 @@ fun MainScaffold(viewModel: ChatViewModel, modifier: Modifier = Modifier) {
         // so the Scaffold must not pad the top. The bottom bar still reports its own height.
         contentWindowInsets = WindowInsets(0),
         bottomBar = {
-            AnimatedVisibility(
-                visible = !inConversation,
-                enter = slideInVertically { it } + fadeIn(),
-                exit = slideOutVertically { it } + fadeOut()
-            ) {
+            // Shown or hidden in the same frame the screen swaps, deliberately not animated. A
+            // slide/fade AnimatedVisibility keeps its full layout slot for the whole exit and only
+            // then collapses to zero, so the conversation opened above a blank strip and lurched
+            // down by the bar's height a few hundred milliseconds later.
+            if (!inConversation) {
                 BitchatNavigationBar(
                     viewModel = viewModel,
                     selected = tab,

app/src/main/java/com/bitchat/android/ui/MessageComponents.kt

@@ -183,8 +183,18 @@ internal fun MessageArrivalTracker.arrivals(messages: List<BitchatMessage>): Set
     val isWholesaleReplacement =
         messages.isNotEmpty() && known.isNotEmpty() && messages.none { it.id in known }
 
+    // History backfill inserts messages *older* than ones already on screen: opening a DM shows its
+    // compacted summary (the latest message only), then loads the stored history in behind it.
+    // Those rows are not arrivals - only messages after the newest already-known one are. Without
+    // this, a short conversation's whole history (anything under the burst cap) slid in on every
+    // open, which read as the transcript flickering.
+    val lastKnownIndex = messages.indexOfLast { it.id in known }
+
     // `HashSet.add` reports whether the id was new, so this both diffs and updates in one pass.
-    val added = messages.filter { known.add(it.id) }
+    // The add runs first so backfilled ids are still recorded as known.
+    val added = messages.filterIndexed { index, message ->
+        known.add(message.id) && index > lastKnownIndex
+    }
 
     if (known.size > messages.size) {
         // Messages disappeared (/clear, channel switch). Drop the stale ids so the set cannot

app/src/test/java/com/bitchat/android/ui/MessageArrivalTrackerTest.kt

@@ -171,6 +171,32 @@ class MessageArrivalTrackerTest {
         assertEquals(setOf("y"), tracker.arrivals(switched))
     }
 
+    @Test
+    fun `history backfilled behind the latest message is adopted silently`() {
+        // Opening a DM shows its compacted summary (just the latest message), then the stored
+        // history loads in behind it. Those older rows are not arrivals; animating them made the
+        // whole transcript flicker on every open of a short conversation.
+        val tracker = MessageArrivalTracker()
+        val latest = msg("latest")
+        tracker.arrivals(listOf(latest))
+
+        val backfilled = listOf(msg("h1"), msg("h2"), msg("h3"), latest)
+        assertTrue(tracker.arrivals(backfilled).isEmpty())
+        assertEquals(setOf("h1", "h2", "h3", "latest"), tracker.known)
+    }
+
+    @Test
+    fun `a new message arriving alongside a backfill still animates`() {
+        val tracker = MessageArrivalTracker()
+        val latest = msg("latest")
+        tracker.arrivals(listOf(latest))
+
+        assertEquals(
+            setOf("new"),
+            tracker.arrivals(listOf(msg("h1"), latest, msg("new")))
+        )
+    }
+
     @Test
     fun `the known set never outgrows the conversation`() {
         val tracker = MessageArrivalTracker()