feat(6839): the decision log and the knowledge feed load as you scroll, through one ScrollSentinel

ScrollSentinel sits at the foot of a server-paged list and asks for the
next page a screen before the reader reaches it. It asks once per cursor,
forgets the cursor when its element goes, and shows Try again on a failed
page instead of retrying in a loop.

FamilyView's decision log loses its "Older decisions" button. A failed
page shows in place rather than as a toast, and rows already shown are
skipped when a new decision shifts the offset.

KnowledgeView drops its inline IntersectionObserver for the component.
The cursor is the ids consumed, not the items rendered, so a batch that
returns fewer records than it asked for still moves the feed on. A failed
batch puts its ids back and offers Try again, where it used to fail
silently and stop.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/components/ScrollSentinel.vue

@@ -0,0 +1,96 @@
+<script setup lang="ts">
+/**
+ * The end of a server-paged list: asks for the next page as the reader nears
+ * it, in place of a "Load more" button. The parent keeps the paging — what the
+ * next page is keyed on, whether one is in flight, whether the last one
+ * failed — and this owns the observing.
+ *
+ *   <ScrollSentinel :cursor="more ? String(rows.length) : ''"
+ *                   :loading="loadingMore" :failed="moreFailed" @more="loadMore" />
+ *
+ * An empty cursor means the list is complete, and nothing renders.
+ */
+import { onBeforeUnmount, ref, watch } from "vue";
+
+const props = defineProps<{ cursor: string; loading: boolean; failed?: boolean }>();
+const emit = defineEmits<{ more: [] }>();
+
+const sentinel = ref<HTMLElement | null>(null);
+const visible = ref(false);
+
+// One request per page. The observer reports every crossing, and a short last
+// page leaves the sentinel on screen for good, so the cursor last asked for is
+// remembered: a page is asked for once, and a failed one is not retried on
+// every scroll, because a failure leaves the cursor where it was. Only
+// Try again asks for it again.
+let asked = "";
+
+function pump() {
+  if (!visible.value || props.loading || props.failed || !props.cursor) return;
+  if (props.cursor === asked) return;
+  asked = props.cursor;
+  emit("more");
+}
+
+function retry() {
+  asked = props.cursor;
+  emit("more");
+}
+
+let observer: IntersectionObserver | undefined;
+
+function observe(el: HTMLElement | null) {
+  observer?.disconnect();
+  observer = undefined;
+  if (!el) {
+    // Visibility and the cursor asked for go with the element. A list that
+    // starts over must not inherit "the end is on screen" from the one it
+    // replaced, and an offset cursor repeats from list to list, so "50" asked
+    // of the old list must not stop the new one asking for its own "50".
+    visible.value = false;
+    asked = "";
+    return;
+  }
+  observer = new IntersectionObserver(
+    (entries) => {
+      visible.value = entries.some((e) => e.isIntersecting);
+      pump();
+    },
+    // Extends the watched area a screen below the viewport, so the next page is
+    // asked for before the reader reaches the end rather than when they do.
+    { rootMargin: "0px 0px 100% 0px" },
+  );
+  observer.observe(el);
+}
+
+// flush: "post": the template ref is only set once the DOM is patched.
+watch(sentinel, observe, { flush: "post" });
+
+// A page that lands without pushing the end off screen gives the observer
+// nothing new to report, so carry on from the props, or the list stops early.
+watch([() => props.cursor, () => props.loading, () => props.failed], pump);
+
+onBeforeUnmount(() => observer?.disconnect());
+</script>
+
+<template>
+  <div v-if="cursor" class="sentinel">
+    <!-- The one case with a button: an observer re-firing on a failure would be
+         a request loop. -->
+    <p v-if="failed" class="sentinel-note sentinel-failed" role="alert">
+      Couldn’t load more.
+      <button type="button" class="btn-secondary btn-sm" @click="retry">Try again</button>
+    </p>
+    <template v-else>
+      <div ref="sentinel" class="sentinel-mark" aria-hidden="true"></div>
+      <p v-if="loading" class="sentinel-note" role="status">Loading more…</p>
+    </template>
+  </div>
+</template>
+
+<style scoped>
+.sentinel { grid-column: 1 / -1; padding: 0.75rem 0; text-align: center; }
+.sentinel-mark { height: 1px; }
+.sentinel-note { margin: 0; font-size: 0.8rem; color: var(--fs-text-tertiary); }
+.sentinel-failed { display: flex; align-items: center; justify-content: center; gap: 0.75rem; }
+</style>

frontend/src/views/FamilyView.vue

@@ -19,6 +19,7 @@ import {
   type FamilyIdea, type IdeaSnapshot, type IdeaStatus,
 } from "@/api/family";
 import FamilyAdoptionMatrix from "@/components/FamilyAdoptionMatrix.vue";
+import ScrollSentinel from "@/components/ScrollSentinel.vue";
 import { useToastStore } from "@/stores/toast";
 import { fmtStamp } from "@/utils/dateFormat";
 import { recordHref } from "@/utils/recordHref";
@@ -36,6 +37,8 @@ const loading = ref(true);
 const failed = ref(false);
 const moreDecisions = ref(false);
 const loadingMore = ref(false);
+const moreFailed = ref(false);
+const decisionOffset = ref(0);
 
 // One pending reversal at a time: which row, which act, and its reason.
 const pending = ref<{ kind: "undo" | "retire"; id: number } | null>(null);
@@ -63,7 +66,9 @@ async function load() {
     criteria.value = ideaData.criteria;
     conflictOrder.value = ideaData.conflict_order ?? [];
     decisions.value = log;
+    decisionOffset.value = log.length;
     moreDecisions.value = log.length === PAGE;
+    moreFailed.value = false;
   } catch {
     // Said out loud: a log that failed to load must not read as "nothing
     // has been decided".
@@ -73,14 +78,20 @@ async function load() {
   }
 }
 
+// Older decisions load as the log is scrolled. The log is offset-paged, so a
+// decision recorded since the first page shifts the rest down by one; a row
+// already shown is skipped, and the offset still counts what the server sent.
 async function loadMore() {
   loadingMore.value = true;
+  moreFailed.value = false;
   try {
-    const next = await listFamilyDecisions(PAGE, decisions.value.length);
-    decisions.value.push(...next);
+    const next = await listFamilyDecisions(PAGE, decisionOffset.value);
+    const shown = new Set(decisions.value.map((d) => d.id));
+    decisions.value.push(...next.filter((d) => !shown.has(d.id)));
+    decisionOffset.value += next.length;
     moreDecisions.value = next.length === PAGE;
-  } catch (e: unknown) {
-    toast.show(apiErrorMessage(e, "Could not load more decisions"), "error");
+  } catch {
+    moreFailed.value = true;
   } finally {
     loadingMore.value = false;
   }
@@ -343,9 +354,12 @@ onMounted(load);
             </form>
           </li>
         </ul>
-        <button v-if="moreDecisions" class="btn-secondary btn-sm" :disabled="loadingMore" @click="loadMore">
-          {{ loadingMore ? "Loading…" : "Older decisions" }}
-        </button>
+        <ScrollSentinel
+          :cursor="moreDecisions ? String(decisionOffset) : ''"
+          :loading="loadingMore"
+          :failed="moreFailed"
+          @more="loadMore"
+        />
       </section>
     </template>
   </main>

frontend/src/views/KnowledgeView.vue

@@ -1,5 +1,5 @@
 <script setup lang="ts">
-import { ref, computed, watch, onMounted, onUnmounted, nextTick } from "vue";
+import { ref, computed, watch, onMounted, onUnmounted } from "vue";
 import { useRouter } from "vue-router";
 import { apiGet } from "@/api/client";
 import type { TaskKind, TaskStatus, TaskPriority } from "@/types/note";
@@ -10,6 +10,7 @@ import UsageBadge from "@/components/UsageBadge.vue";
 import NoteSweepPane from "@/components/NoteSweepPane.vue";
 import StatusBadge from "@/components/StatusBadge.vue";
 import PriorityBadge from "@/components/PriorityBadge.vue";
+import ScrollSentinel from "@/components/ScrollSentinel.vue";
 import GraphView from "@/views/GraphView.vue";
 import {
   FileText,
@@ -193,7 +194,7 @@ function onClickOutsideNewNote(e: MouseEvent) {
 
 const ID_BATCH = 100;         // IDs fetched per server round-trip
 const INITIAL_CONTENT = 50;   // items loaded on first render
-const CONTENT_PAGE = 24;      // items loaded per sentinel trigger
+const CONTENT_PAGE = 24;      // items loaded each time the end of the feed nears
 const REFILL_THRESHOLD = 48;  // fetch more IDs when queue drops below this
 
 const items = ref<KnowledgeItem[]>([]);
@@ -214,11 +215,21 @@ const totalKnowledge = ref(0);       // total matching items on server
 const allIdsFetched = ref(false);    // no more ID pages to fetch
 const idsFetching = ref(false);      // ID batch request in-flight
 const contentFetching = ref(false);  // content batch request in-flight
-const sentinelVisible = ref(false);  // updated by IntersectionObserver
+const moreFailed = ref(false);       // the last page asked for did not load
 
 const loading = computed(() => idsFetching.value || contentFetching.value);
 
 let fetchGen = 0;  // incremented on each reset to invalidate stale responses
+const feedGen = ref(0);  // fetchGen, readable by the template
+
+// What the next page is keyed on: the feed it belongs to and how many of its ids
+// have been loaded. Ids, not items: a batch can return fewer records than it
+// asked for, and the feed must still move on. Empty once every id is loaded.
+const nextCursor = computed(() =>
+  idQueue.value.length > 0 || !allIdsFetched.value
+    ? `${feedGen.value}:${idOffset.value - idQueue.value.length}`
+    : "",
+);
 
 function buildFilterParams(): URLSearchParams {
   const p = new URLSearchParams();
@@ -244,7 +255,9 @@ async function fetchIdBatch(gen: number) {
     idOffset.value += data.ids.length;
     totalKnowledge.value = data.total;
     if (!data.has_more) allIdsFetched.value = true;
-  } catch { /* silent */ }
+  } catch {
+    if (gen === fetchGen) moreFailed.value = true;
+  }
   finally { if (gen === fetchGen) idsFetching.value = false; }
 }
 
@@ -262,26 +275,38 @@ async function loadNextContent(gen: number, count: number) {
     if (idQueue.value.length < REFILL_THRESHOLD && !allIdsFetched.value && !idsFetching.value) {
       fetchIdBatch(gen);
     }
-  } catch { /* silent */ }
-  finally {
-    if (gen !== fetchGen) return;
-    contentFetching.value = false;
-    // Sentinel may still be visible if new items didn't push it off screen
-    await nextTick();
-    if (sentinelVisible.value && idQueue.value.length > 0 && gen === fetchGen) {
-      loadNextContent(gen, CONTENT_PAGE);
+  } catch {
+    // The ids go back to the front of the queue, so Try again asks for the
+    // same records rather than skipping past them.
+    if (gen === fetchGen) {
+      idQueue.value.unshift(...toLoad);
+      moreFailed.value = true;
     }
   }
+  finally {
+    if (gen === fetchGen) contentFetching.value = false;
+  }
+}
+
+// The next page of the feed: more ids first when the queue has run dry.
+async function loadMore() {
+  const gen = fetchGen;
+  moreFailed.value = false;
+  if (idQueue.value.length === 0) await fetchIdBatch(gen);
+  if (gen !== fetchGen) return;
+  await loadNextContent(gen, CONTENT_PAGE);
 }
 
 async function reset() {
   fetchGen++;
+  feedGen.value = fetchGen;
   const gen = fetchGen;
   items.value = [];
   idQueue.value = [];
   idOffset.value = 0;
   totalKnowledge.value = 0;
   allIdsFetched.value = false;
+  moreFailed.value = false;
   await fetchIdBatch(gen);
   if (gen !== fetchGen) return;
   await loadNextContent(gen, INITIAL_CONTENT);
@@ -294,23 +319,16 @@ async function fetchTags() {
   } catch { /* silent */ }
 }
 
-async function resetAndReobserve() {
-  await reset();
-  await nextTick();
-  setupObserver();
-}
-
 function onSearchInput() {
   if (searchDebounce) clearTimeout(searchDebounce);
-  searchDebounce = setTimeout(() => resetAndReobserve(), 380);
+  searchDebounce = setTimeout(() => reset(), 380);
 }
 
-watch([activeType, sortMode], () => resetAndReobserve());
-// Closing the sweep remounts the feed, and with it the scroll sentinel — a
-// fresh element the old observer is not watching. Without this the list loads
-// its first page and then never loads another.
-watch(sweepActive, (open) => { if (!open) resetAndReobserve(); });
-watch(activeTag, () => { fetchCounts(); resetAndReobserve(); });
+watch([activeType, sortMode], () => reset());
+// Closing the sweep starts the feed over, so a note confirmed in the sweep is
+// shown as it now stands.
+watch(sweepActive, (open) => { if (!open) reset(); });
+watch(activeTag, () => { fetchCounts(); reset(); });
 
 // ─── Today bar ────────────────────────────────────────────────────────────────
 
@@ -377,24 +395,6 @@ function formatDate(iso: string): string {
   return d.toLocaleDateString(undefined, { month: "short", day: "numeric", year: "numeric" });
 }
 
-// ─── Sentinel observer ────────────────────────────────────────────────────────
-
-const sentinelEl = ref<HTMLElement | null>(null);
-let observer: IntersectionObserver | null = null;
-
-function setupObserver() {
-  observer?.disconnect();
-  if (!sentinelEl.value) return;
-  observer = new IntersectionObserver(
-    ([entry]) => {
-      sentinelVisible.value = entry.isIntersecting;
-      if (entry.isIntersecting) loadNextContent(fetchGen, CONTENT_PAGE);
-    },
-    { root: null, rootMargin: "200px" }
-  );
-  observer.observe(sentinelEl.value);
-}
-
 // ─── Lifecycle ────────────────────────────────────────────────────────────────
 
 onMounted(async () => {
@@ -403,14 +403,11 @@ onMounted(async () => {
   fetchTags();
   fetchCounts();
   fetchTodayBar();
-  await nextTick();
-  setupObserver();
 });
 
 onUnmounted(() => {
   document.removeEventListener('click', onClickOutsideNewNote);
   if (searchDebounce) clearTimeout(searchDebounce);
-  observer?.disconnect();
 });
 </script>
 
@@ -583,10 +580,7 @@ onUnmounted(() => {
         </div>
 
         <!-- The whole feed stands down while the sweep is open: two answers
-             to two different questions on one screen is neither. Wrapped
-             rather than given an extra v-if branch, because the scroll
-             sentinel lives inside the grid and the observer must not be left
-             holding a ref to something that never renders. -->
+             to two different questions on one screen is neither. -->
         <template v-if="!sweepActive">
         <!-- Loading / empty -->
         <div v-if="loading && items.length === 0" class="knowledge-empty">Loading…</div>
@@ -675,10 +669,12 @@ onUnmounted(() => {
             </div>
           </div>
 
-          <!-- Sentinel — IntersectionObserver triggers next content batch -->
-          <div ref="sentinelEl" class="scroll-sentinel">
-            <span v-if="contentFetching" class="sentinel-loading">Loading…</span>
-          </div>
+          <ScrollSentinel
+            :cursor="nextCursor"
+            :loading="loading"
+            :failed="moreFailed"
+            @more="loadMore"
+          />
         </div>
         </template>
       </div>
@@ -1115,19 +1111,6 @@ onUnmounted(() => {
   opacity: 0.85;
 }
 
-/* ── Sentinel ────────────────────────────────────────────── */
-.scroll-sentinel {
-  grid-column: 1 / -1;
-  height: 40px;
-  display: flex;
-  align-items: center;
-  justify-content: center;
-}
-.sentinel-loading {
-  font-size: 0.8rem;
-  color: var(--fs-text-tertiary);
-}
-
 /* ── Graph panel ─────────────────────────────────────────── */
 .graph-panel {
   width: 500px;