diff --git a/apps/server/src/services/threads/timeline.ts b/apps/server/src/services/threads/timeline.ts
index 430d64660..ccd2379d9 100644
--- a/apps/server/src/services/threads/timeline.ts
+++ b/apps/server/src/services/threads/timeline.ts
@@ -1538,13 +1538,13 @@ function buildSequencePageTimelineRows(
     selection.responsePageKind === "latest"
       ? ""
       : `:sequence-page:${selection.byteWindowSequenceStart}`;
-  return rowsWithPlaceholder.map((row): TimelineRow => {
+  return rowsWithPlaceholder.flatMap((row): TimelineRow[] => {
     if (
       row.kind !== "turn" ||
       selection.byteWindowSequenceEnd === null ||
       selection.byteWindowSequenceStart === null
     ) {
-      return { ...row, id: `${row.id}${suffix}` };
+      return [{ ...row, id: `${row.id}${suffix}` }];
     }
     const sourceSeqStart = Math.max(
       row.sourceSeqStart,
@@ -1555,13 +1555,18 @@ function buildSequencePageTimelineRows(
       selection.byteWindowSequenceEnd,
     );
     return sourceSeqStart <= sourceSeqEnd
-      ? {
-          ...row,
-          id: `${row.id}${suffix}`,
-          sourceSeqEnd,
-          sourceSeqStart,
-        }
-      : { ...row, id: `${row.id}${suffix}` };
+      ? [
+          {
+            ...row,
+            id: `${row.id}${suffix}`,
+            sourceSeqEnd,
+            sourceSeqStart,
+          },
+        ]
+      : // PROTOTYPE (#1868): a finished-turn row whose events lie entirely
+        // outside this byte window came in through parent/lifecycle closure
+        // and belongs to the page that actually holds those events.
+        [];
   });
 }
 
diff --git a/packages/db/src/data/events.ts b/packages/db/src/data/events.ts
index e7de63b63..061d9de86 100644
--- a/packages/db/src/data/events.ts
+++ b/packages/db/src/data/events.ts
@@ -2838,6 +2838,19 @@ function storedTimelineWindowConditions(
   if (args.excludedTypes && args.excludedTypes.length > 0) {
     conditions.push(notInArray(events.type, [...args.excludedTypes]));
   }
+  // PROTOTYPE (#1868): superseded background-task progress snapshots are
+  // never load-bearing (see pruneBackgroundTaskProgressEvents), so neither
+  // the byte budget nor the materialized window should include them.
+  const progressType =
+    "item/backgroundTask/progress" satisfies ThreadEventType;
+  conditions.push(
+    sql`(${events.type} <> ${progressType} OR ${events.id} IN (
+      SELECT latest.id FROM events latest
+      WHERE latest.thread_id = ${events.threadId}
+        AND latest.type = ${progressType}
+        AND latest.item_id = ${events.itemId}
+      ORDER BY latest.sequence DESC LIMIT 1))`,
+  );
   return conditions;
 }