ui: cap the eased scroll and drop the snapshot focus map

A system trace on the TV showed the eased focus scroll re-ticking for
hundreds of milliseconds per move — animateScrollToItem's default spec
scales with distance, and every tick redraws the whole window on a TV
GPU, with 145 ms spikes when new cards composed mid-animation. That is
what drags row scrolling back down to a few fps, again (same symptom
51077b8 and 5ea1ca5 each fixed a facet of). easedScrollToItem keeps the
eased feel but caps the tween at ~140 ms, so a move costs a handful of
cheap ticks instead of hundreds.

The per-card focus requesters in the video grid were back in a snapshot
state map written during composition — the exact recompose loop
5ea1ca5 fixed in the rows, still alive in the grid screens. Plain
HashMap again, keyed on epoch only: keyed on items.size the map was
dropped wholesale on every page load, and already-composed cards didn't
re-populate it until their next recompose.

Co-Authored-By: Claude <noreply@anthropic.com>

app/src/main/java/net/jeena/jtubetv/ui/screens/ChannelsScreen.kt

@@ -2,6 +2,8 @@ package net.jeena.jtubetv.ui.screens
 
 import androidx.compose.foundation.BorderStroke
 import androidx.compose.foundation.background
+import androidx.compose.animation.core.tween
+import androidx.compose.foundation.gestures.animateScrollBy
 import androidx.compose.foundation.layout.Arrangement
 import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.Column
@@ -469,7 +471,7 @@ private fun ChannelRow(
             val notFullyVisible = target == null ||
                 target.offset < 0 ||
                 target.offset + target.size > rowState.layoutInfo.viewportEndOffset
-            if (notFullyVisible) rowState.animateScrollToItem(focusedIndex)
+            if (notFullyVisible) rowState.easedScrollToItem(focusedIndex)
         }
     }
     if (focused) {
@@ -574,13 +576,19 @@ fun ChannelCard(
                 val coverUrl = banner?.let { absoluteBanner(it) }
                     ?: stats?.latestThumbUrl
                 if (coverUrl != null) {
-                    ShimmerAsyncImage(
-                        model = coil.request.ImageRequest.Builder(
-                            androidx.compose.ui.platform.LocalContext.current,
-                        )
+                    // The request must be remembered: ImageRequest has no value equality,
+                    // a fresh request per recomposition reset the shimmer (and retried
+                    // the load) every frame — the fps regression on the TV.
+                    val ctx = androidx.compose.ui.platform.LocalContext.current
+                    val coverKey = remember(channel.name, channel.host) { channel.name + "@" + channel.host }
+                    val coverReq = remember(coverKey, coverUrl) {
+                        coil.request.ImageRequest.Builder(ctx)
                             .data(coverUrl)
                             .size(480, 270) // same key as prefetch
-                            .build(),
+                            .build()
+                    }
+                    ShimmerAsyncImage(
+                        model = coverReq,
                         contentDescription = channel.displayName,
                         contentScale = ContentScale.Crop,
                         modifier = Modifier.fillMaxWidth().aspectRatio(16f / 9f),
@@ -588,13 +596,18 @@ fun ChannelCard(
                 }
                 val avatar = channel.avatars.maxByOrNull { it.width }
                 if (avatar != null) {
-                    ShimmerAsyncImage(
-                        model = coil.request.ImageRequest.Builder(
-                            androidx.compose.ui.platform.LocalContext.current,
+                    val ctx = androidx.compose.ui.platform.LocalContext.current
+                    val avatarUrl = remember(avatar.path, avatar.fileUrl) { absoluteAvatar(avatar) }
+                    val avatarReq = remember(avatarUrl) {
+                        coil.request.ImageRequest.Builder(
+                            ctx,
                         )
-                            .data(absoluteAvatar(avatar))
+                            .data(avatarUrl)
                             .size(80, 80)
-                            .build(),
+                            .build()
+                    }
+                    ShimmerAsyncImage(
+                        model = avatarReq,
                         contentDescription = null,
                         contentScale = ContentScale.Crop,
                         base = Color(0xFF333333),

app/src/main/java/net/jeena/jtubetv/ui/screens/HomeRows.kt

@@ -1,5 +1,7 @@
 package net.jeena.jtubetv.ui.screens
 
+import androidx.compose.animation.core.tween
+import androidx.compose.foundation.gestures.animateScrollBy
 import androidx.compose.foundation.layout.Arrangement
 import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.Column
@@ -9,6 +11,7 @@ import androidx.compose.foundation.layout.fillMaxSize
 import androidx.compose.foundation.layout.padding
 import androidx.compose.foundation.layout.width
 import androidx.compose.foundation.lazy.LazyColumn
+import androidx.compose.foundation.lazy.LazyListState
 import androidx.compose.foundation.lazy.LazyRow
 import androidx.compose.foundation.lazy.itemsIndexed
 import androidx.compose.foundation.lazy.rememberLazyListState
@@ -450,7 +453,7 @@ fun FocusScrollRow(
             val notFullyVisible = target == null ||
                 target.offset < 0 ||
                 target.offset + (target.size / 2) > rowState.layoutInfo.viewportEndOffset
-            if (notFullyVisible) rowState.animateScrollToItem(focusedIndex)
+            if (notFullyVisible) rowState.easedScrollToItem(focusedIndex)
         }
     }
     LazyRow(
@@ -495,3 +498,19 @@ fun FocusScrollRow(
         }
     }
 }
+
+/**
+ * Eased scroll to [index] with a capped duration (perf): [animateScrollToItem]'s
+ * default spec keeps re-ticking for hundreds of ms on long distances, and every
+ * tick is a full-window redraw on a TV GPU — which is what dragged the scroll
+ * down to a few fps. A short tween keeps the eased feel at a fraction of the
+ * ticks. (Shared by the home rows and the channels rows.)
+ */
+suspend fun LazyListState.easedScrollToItem(index: Int) {
+    val info = layoutInfo
+    val first = info.visibleItemsInfo.firstOrNull() ?: return
+    val stride = first.size + info.mainAxisItemSpacing
+    if (stride <= 0) return
+    val delta = index * stride - (first.index * stride + firstVisibleItemScrollOffset)
+    animateScrollBy(delta.toFloat(), tween(140))
+}

app/src/main/java/net/jeena/jtubetv/ui/screens/list/VideoCard.kt

@@ -124,11 +124,18 @@ fun VideoCard(
                 .background(Color(0xFF222222)),
         ) {
             video.thumbnailPath?.takeIf { it.isNotBlank() }?.let { path ->
-                val req = coil.request.ImageRequest.Builder(androidx.compose.ui.platform.LocalContext.current)
+                // The request object must be remembered: ImageRequest has no value
+                // equality, and a fresh request per recomposition reset the
+                // shimmer's loaded state (and retried the load) on every frame —
+                // the fps regression on the TV.
+                val ctx = androidx.compose.ui.platform.LocalContext.current
+                val req = remember(video.id, video.thumbnailUrl) {
+                    coil.request.ImageRequest.Builder(ctx)
                         // search-index results carry an absolute URL for their own instance
                         .data(video.thumbnailUrl ?: AppGraph.repository.absoluteUrl(path))
                         .size(480, 270) // ~2x the drawn thumb; avoids decoding 1080p+ jpegs
                         .build()
+                }
                 ShimmerAsyncImage(
                     model = req,
                     contentDescription = video.name,

app/src/main/java/net/jeena/jtubetv/ui/screens/list/VideoListScreen.kt

@@ -74,9 +74,14 @@ fun VideoListScreen(
     ),
 ) {
     val items = viewModel.items
-    // per-item focus requesters so BACK from the player can refocus the opened card
-    val requesters = androidx.compose.runtime.remember(items.size, epoch) {
-        androidx.compose.runtime.mutableStateMapOf<Int, FocusRequester>()
+    // per-item focus requesters so BACK from the player can refocus the opened card.
+    // A plain HashMap, like the rows: this is/was written during composition of
+    // every card, and a snapshot map here re-invalidated the reading scopes on
+    // each write — the exact recompose loop behind the old 2fps scroll (5ea1ca5).
+    // Keyed only on [epoch]: on items.size the map was dropped wholesale, and
+    // already-composed cards didn't re-populate it until their next recompose.
+    val requesters = androidx.compose.runtime.remember(epoch) {
+        HashMap<Int, FocusRequester>()
     }
     // Scroll position survives destination switches (per source), not just player exits.
     val gridState = androidx.compose.runtime.saveable.rememberSaveable(
@@ -92,10 +97,14 @@ fun VideoListScreen(
         // handing the first card the focus animates the grid up (a focused card
         // that sits partly off-screen is pulled into view), and this effect then
         // snapped it back down — the jump that was still reported on the channel
-        // page when the first video got focus (#27 review). Doing the park first,
-        // and only while the grid is actually displaced, leaves nothing to undo.
+        // page when the first video got focus (#27 review).
+        // The park has to be unconditional, and has to reset the row's offset,
+        // not just its index: the focus pull leaves the header row at index 0 but
+        // slid up by ~96 dp, so a guard on firstVisibleItemIndex > 0 never fires
+        // in that state and the top of the channel header stays clipped off
+        // screen (#31 review). scrollToItem(0) re-anchors both.
         LaunchedEffect(items.isNotEmpty(), source) {
-            if (items.isNotEmpty() && gridState.firstVisibleItemIndex > 0) {
+            if (items.isNotEmpty()) {
                 runCatching { gridState.scrollToItem(0) }
             }
         }
@@ -436,12 +445,14 @@ private fun ChannelHeader(
             .clip(androidx.compose.foundation.shape.RoundedCornerShape(12.dp))
             .background(Color.Black),
     ) {
-    // Banner area: a fixed height with the identity + stats anchored to its
-    // bottom; the description sits in the black strip below it.
+    // Banner area: 116 dp, with the identity + stats anchored to its bottom; the
+    // description sits in the black strip below it. The whole band is 160 dp
+    // (4 + 116 + 40) — under one card row — so the grid never reflows around it
+    // and the band stays fully on screen when the first card takes focus (#31).
     Box(
         Modifier
             .fillMaxWidth()
-            .height(178.dp)
+            .height(116.dp)
             .background(androidx.compose.ui.graphics.Brush.linearGradient(accent)),
     ) {
         if (banner != null) {
@@ -563,7 +574,7 @@ private fun ChannelHeader(
     Box(
         Modifier
             .fillMaxWidth()
-            .height(42.dp)
+            .height(40.dp)
             .padding(start = 16.dp, end = 16.dp, top = 6.dp),
     ) {
         if (description != null) {