fix(chat): prevent timeline prefetch ANRs (#985)

app/src/main/java/com/m57/hermescontrol/ui/chat/ChatScreen.kt

@@ -92,6 +92,7 @@ import com.m57.hermescontrol.ui.chat.components.ChatLifecycleEffects
 import com.m57.hermescontrol.ui.chat.components.ChatLoadingOverlay
 import com.m57.hermescontrol.ui.chat.components.ChatResumeErrorOverlay
 import com.m57.hermescontrol.ui.chat.components.ChatScrollToBottomFab
+import com.m57.hermescontrol.ui.chat.components.ChatTimelineNoPrefetchStrategy
 import com.m57.hermescontrol.ui.chat.components.ContextDetailSheet
 import com.m57.hermescontrol.ui.chat.components.ContextUsageChip
 import com.m57.hermescontrol.ui.chat.components.ReactionHeartsOverlay
@@ -158,7 +159,7 @@ fun ChatScreen(
     // read its fields recompose on search changes (bar, matched bubbles).
     val searchState = viewModel.searchState
     val lifecycleOwner = LocalLifecycleOwner.current
-    val listState = rememberLazyListState()
+    val listState = rememberLazyListState(prefetchStrategy = ChatTimelineNoPrefetchStrategy)
     val scrollScope = rememberCoroutineScope()
     val scrollController = rememberChatScrollController(listState, scrollScope)
     var isOlderPagingArmed by remember(state.currentSessionId) { mutableStateOf(false) }

app/src/main/java/com/m57/hermescontrol/ui/chat/components/ChatTimelineNoPrefetchStrategy.kt

@@ -0,0 +1,25 @@
+package com.m57.hermescontrol.ui.chat.components
+
+import androidx.compose.foundation.ExperimentalFoundationApi
+import androidx.compose.foundation.lazy.LazyListLayoutInfo
+import androidx.compose.foundation.lazy.LazyListPrefetchScope
+import androidx.compose.foundation.lazy.LazyListPrefetchStrategy
+import androidx.compose.foundation.lazy.layout.NestedPrefetchScope
+
+/**
+ * Chat messages can contain very large selectable Markdown blocks. Measuring a
+ * neighboring item during LazyColumn prefetch can monopolize the main thread
+ * long enough to trigger an input-dispatch ANR, even though that item is still
+ * off screen. Measure chat items only when they enter the viewport instead.
+ */
+@OptIn(ExperimentalFoundationApi::class)
+internal object ChatTimelineNoPrefetchStrategy : LazyListPrefetchStrategy {
+    override fun LazyListPrefetchScope.onScroll(
+        delta: Float,
+        layoutInfo: LazyListLayoutInfo,
+    ) = Unit
+
+    override fun LazyListPrefetchScope.onVisibleItemsUpdated(layoutInfo: LazyListLayoutInfo) = Unit
+
+    override fun NestedPrefetchScope.onNestedPrefetch(firstVisibleItemIndex: Int) = Unit
+}

app/src/test/java/com/m57/hermescontrol/ui/chat/components/ChatTimelinePrefetchStrategyTest.kt

@@ -0,0 +1,33 @@
+package com.m57.hermescontrol.ui.chat.components
+
+import androidx.compose.foundation.ExperimentalFoundationApi
+import androidx.compose.foundation.lazy.LazyListLayoutInfo
+import androidx.compose.foundation.lazy.LazyListPrefetchScope
+import androidx.compose.foundation.lazy.layout.NestedPrefetchScope
+import io.mockk.mockk
+import io.mockk.verify
+import org.junit.Test
+
+@OptIn(ExperimentalFoundationApi::class)
+class ChatTimelinePrefetchStrategyTest {
+    @Test
+    fun strategy_neverSchedulesOffscreenChatItems() {
+        val scope = mockk<LazyListPrefetchScope>(relaxed = true)
+        val layoutInfo = mockk<LazyListLayoutInfo>(relaxed = true)
+        val nestedScope = mockk<NestedPrefetchScope>(relaxed = true)
+
+        with(ChatTimelineNoPrefetchStrategy) {
+            with(scope) {
+                onScroll(delta = 120f, layoutInfo)
+                onVisibleItemsUpdated(layoutInfo)
+            }
+            with(nestedScope) {
+                onNestedPrefetch(firstVisibleItemIndex = 10)
+            }
+        }
+
+        verify(exactly = 0) { scope.schedulePrefetch(any(), any()) }
+        verify(exactly = 0) { nestedScope.schedulePrecomposition(any()) }
+        verify(exactly = 0) { nestedScope.schedulePrecompositionAndPremeasure(any(), any()) }
+    }
+}