perf(panel): cached info and history paint first, lazy markdown up the thread, background prefetch

ui/src/lib/gateway.svelte.ts

@@ -5,6 +5,7 @@ import type {
   LimitWindow,
 } from "./api/types";
 import { panelCache } from "./panel/cache";
+import { browserHost } from "./panel/host";
 import { ConversationIndex } from "./panel/index.svelte";
 import { session } from "./session.svelte";
 
@@ -23,7 +24,7 @@ class Gateway extends ConversationIndex {
   jobs = $state<JobsResponse | null>(cache.get<JobsResponse>(JOBS_KEY) ?? null);
 
   constructor() {
-    super(() => session.client);
+    super(() => session.client, browserHost().cache ?? null);
   }
 
   async refreshJobs(): Promise<void> {

ui/src/lib/panel/chat-view.svelte

@@ -8,6 +8,7 @@
   import { cn } from "$lib/utils";
   import type { ActivityModel } from "./activity.svelte";
   import { summarizeInput, toolLabel } from "./activity.svelte";
+  import { cacheableHistory, historyKey } from "./history-cache";
   import { usePanelHost } from "./host";
   import Markdown from "./markdown.svelte";
   import QuestionCard from "./question-card.svelte";
@@ -26,7 +27,9 @@
   } = $props();
 
   const RESULT_CLIP = 600;
-  const CACHED_MESSAGES = 80;
+  // Markdown for the last few screens renders at once; the rest waits
+  // until scrolled near, so a long thread paints as fast as a short one.
+  const EAGER_MARKDOWN = 40;
   const host = usePanelHost();
   const NEAR_BOTTOM_PX = 120;
   const TICK_MS = 1000;
@@ -59,7 +62,7 @@
     failure = null;
     if (messages === null) {
       const cached = host.cache?.get<HistoryMessage[]>(
-        `history:${conversationId}`
+        historyKey(conversationId)
       );
       if (cached) {
         messages = cached;
@@ -69,10 +72,7 @@
       ({ messages } = await client.history(conversationId));
       loadedAt = new Date().toISOString();
       settle();
-      host.cache?.set(
+      host.cache?.set(historyKey(conversationId), cacheableHistory(messages));
-        `history:${conversationId}`,
-        messages.slice(-CACHED_MESSAGES)
-      );
     } catch (cause) {
       failure = cause instanceof Error ? cause.message : String(cause);
     }
@@ -181,6 +181,12 @@
     openSystem = next;
   }
 
+  // Keys count from the end: when the network copy grows the head past
+  // what the cache held, the tail keeps its nodes and its rendered markdown.
+  function keyOf(message: HistoryMessage, index: number, total: number) {
+    return `${index - total}:${message.role}:${message.ts ?? ""}`;
+  }
+
   function blocks(message: HistoryMessage): ContentBlock[] {
     return typeof message.content === "string"
       ? [{ text: message.content, type: "text" }]
@@ -237,7 +243,7 @@
         title="Empty conversation"
       />
     {:else}
-      {#each messages as message, index (index)}
+      {#each messages as message, index (keyOf(message, index, messages.length))}
         {@const parts = blocks(message)}
         {@const isResultOnly = parts.every((b) => b.type === "tool_result")}
         {@const system = systemText(message)}
@@ -303,6 +309,7 @@
                       ? "rounded-2xl rounded-br-md bg-primary/10 px-3.5 py-2"
                       : "px-1 py-1"
                   )}
+                  eager={index >= messages.length - EAGER_MARKDOWN}
                   text={block.text}
                 />
               {:else if block.type === "tool_use"}

ui/src/lib/panel/conversation-view.svelte

@@ -8,6 +8,7 @@
   import ContextView from "./context-view.svelte";
   import { ConversationFeed } from "./conversation.svelte";
   import ConversationHeader from "./conversation-header.svelte";
+  import { usePanelHost } from "./host";
   import RawEntries from "./raw-entries.svelte";
 
   let {
@@ -28,9 +29,11 @@
     initialView?: string;
   } = $props();
 
+  const host = usePanelHost();
   const feed = new ConversationFeed(
     () => client,
-    untrack(() => id)
+    untrack(() => id),
+    host.cache ?? null
   );
   let view = $state(untrack(() => initialView));
   let historyKey = $state(0);

ui/src/lib/panel/conversation.svelte.ts

@@ -1,6 +1,9 @@
 import type { ApiClient } from "$lib/api/client";
 import { LiveStream } from "$lib/api/live.svelte";
+import type { ConversationInfo } from "$lib/api/types";
 import { ActivityModel } from "./activity.svelte";
+import type { PanelCache } from "./cache";
+import { infoKey } from "./history-cache";
 
 // One conversation, live: the snapshot from ``GET /api/conversations/{id}``
 // (bindings, queue, the turn in flight) plus its SSE stream folded into an
@@ -12,10 +15,20 @@ export class ConversationFeed {
   readonly id: string;
   readonly live: LiveStream;
   private readonly client: () => ApiClient | null;
+  private readonly cache: PanelCache | null;
 
-  constructor(client: () => ApiClient | null, id: string) {
+  constructor(
+    client: () => ApiClient | null,
+    id: string,
+    cache: PanelCache | null = null
+  ) {
     this.client = client;
     this.id = id;
+    this.cache = cache;
+    const known = cache?.get<ConversationInfo>(infoKey(id));
+    if (known) {
+      this.model.setConversation(known);
+    }
     this.live = new LiveStream(client, `/api/conversations/${id}/events`, {
       onEvent: (event) => this.model.apply(event),
       prepare: () => this.refresh(),
@@ -28,7 +41,9 @@ export class ConversationFeed {
       return;
     }
     try {
-      this.model.setConversation(await client.conversation(this.id));
+      const info = await client.conversation(this.id);
+      this.model.setConversation(info);
+      this.cache?.set(infoKey(this.id), info);
       this.error = null;
     } catch (error) {
       this.error = error instanceof Error ? error.message : String(error);

ui/src/lib/panel/history-cache.ts

@@ -0,0 +1,28 @@
+import type { ContentBlock, HistoryMessage } from "$lib/api/types";
+
+const MAX_BYTES = 250_000;
+const TAIL = 200;
+
+function slim(message: HistoryMessage): HistoryMessage {
+  if (typeof message.content === "string") {
+    return message;
+  }
+  const content: ContentBlock[] = message.content.map((block) =>
+    block.type === "tool_result" ? { ...block, content: "" } : block
+  );
+  return { ...message, content };
+}
+
+// The tail of a thread as it is worth keeping between opens: tool results
+// dropped (hidden by default, and the bulk of the bytes), then trimmed from
+// the head until it fits a slice of localStorage.
+export function cacheableHistory(messages: HistoryMessage[]): HistoryMessage[] {
+  let tail = messages.slice(-TAIL).map(slim);
+  while (tail.length > 1 && JSON.stringify(tail).length > MAX_BYTES) {
+    tail = tail.slice(Math.ceil(tail.length / 4));
+  }
+  return tail;
+}
+
+export const historyKey = (id: string) => `history:${id}`;
+export const infoKey = (id: string) => `info:${id}`;

ui/src/lib/panel/index.svelte.ts

@@ -2,11 +2,16 @@ import type { ApiClient } from "$lib/api/client";
 import { LiveStream } from "$lib/api/live.svelte";
 import type { BusEvent, ConversationSummary } from "$lib/api/types";
 import type { PanelCache } from "./cache";
+import { cacheableHistory, historyKey, infoKey } from "./history-cache";
 
 const RELOAD_DEBOUNCE_MS = 1500;
 const PAGE = 500;
 const CACHED_ROWS = 200;
 const INDEX_KEY = "index";
+const WARM_TTL_MS = 60_000;
+const WARM_GAP_MS = 120;
+const WARM_AHEAD = 8;
+const QUIET_KINDS = new Set(["job", "fork"]);
 
 // The conversation index kept fresh by ``/api/events``: rows land as
 // ``conversation.*`` events arrive, turn markers flip ``running_turn``
@@ -19,6 +24,8 @@ export class ConversationIndex {
   protected readonly client: () => ApiClient | null;
   private readonly cache: PanelCache | null;
   private reloadTimer: ReturnType<typeof setTimeout> | null = null;
+  private readonly warmed = new Map<string, number>();
+  private warming: Promise<void> | null = null;
 
   constructor(client: () => ApiClient | null, cache: PanelCache | null = null) {
     this.client = client;
@@ -55,6 +62,64 @@ export class ConversationIndex {
     this.conversations = list.conversations;
     this.loaded = true;
     this.cache?.set(INDEX_KEY, list.conversations.slice(0, CACHED_ROWS));
+    this.prefetch(this.likely());
+  }
+
+  // What the operator opens next: anything alive, then the freshest strips.
+  private likely(): string[] {
+    const rows = this.conversations.filter(
+      (row) => !QUIET_KINDS.has(row.kind) && row.status === "open"
+    );
+    const alive = rows.filter(
+      (row) => row.running_turn || row.pending_question
+    );
+    const rest = rows
+      .filter((row) => !(row.running_turn || row.pending_question))
+      .sort((a, b) =>
+        (b.last_activity_at ?? b.created_at ?? "").localeCompare(
+          a.last_activity_at ?? a.created_at ?? ""
+        )
+      );
+    return [...alive, ...rest].slice(0, WARM_AHEAD).map((row) => row.id);
+  }
+
+  // Warm the cache for these threads, one at a time, in the background.
+  prefetch(ids: string[]): void {
+    if (!this.cache) {
+      return;
+    }
+    const now = Date.now();
+    const fresh = ids.filter(
+      (id) => now - (this.warmed.get(id) ?? 0) > WARM_TTL_MS
+    );
+    if (fresh.length === 0) {
+      return;
+    }
+    for (const id of fresh) {
+      this.warmed.set(id, now);
+    }
+    const run = async () => {
+      for (const id of fresh) {
+        const client = this.client();
+        if (!client) {
+          return;
+        }
+        try {
+          // biome-ignore lint/performance/noAwaitInLoops: one thread at a time, on purpose - background warmth, not a race
+          const [history, info] = await Promise.all([
+            client.history(id),
+            client.conversation(id),
+          ]);
+          this.cache?.set(historyKey(id), cacheableHistory(history.messages));
+          this.cache?.set(infoKey(id), info);
+        } catch {
+          this.warmed.delete(id);
+        }
+        // biome-ignore lint/performance/noAwaitInLoops: the pause between fetches is the point
+        await new Promise((resolve) => setTimeout(resolve, WARM_GAP_MS));
+      }
+    };
+    this.warming = (this.warming ?? Promise.resolve()).then(run, run);
   }
 
   apply(event: BusEvent): void {

ui/src/lib/panel/markdown.svelte

@@ -3,8 +3,13 @@
   import { usePanelHost } from "./host";
   import { linkOf, renderMarkdown } from "./markdown";
 
-  let { text, class: className = "" }: { text: string; class?: string } =
+  let {
-    $props();
+    text,
+    class: className = "",
+    eager = true,
+  }: { text: string; class?: string; eager?: boolean } = $props();
+
+  const NEAR = "1200px";
 
   const host = usePanelHost();
   const html = $derived(host.markdown ? "" : renderMarkdown(text));
@@ -36,15 +41,52 @@
     };
   }
 
+  // The host renders when asked; far up the thread that is when the
+  // node comes near the viewport, with the plain text holding its place.
   function render(node: HTMLElement, value: string) {
-    let dispose = host.markdown?.(node, value);
+    let dispose: (() => void) | undefined;
+    let watcher: IntersectionObserver | null = null;
+    let pending = value;
+    const now = (next: string) => {
+      dispose?.();
+      node.classList.remove("whitespace-pre-wrap");
+      dispose = host.markdown?.(node, next);
+    };
+    const later = (next: string) => {
+      pending = next;
+      node.textContent = next;
+      node.classList.add("whitespace-pre-wrap");
+      if (watcher) {
+        return;
+      }
+      watcher = new IntersectionObserver(
+        (entries) => {
+          if (entries.some((entry) => entry.isIntersecting)) {
+            watcher?.disconnect();
+            watcher = null;
+            now(pending);
+          }
+        },
+        { rootMargin: NEAR }
+      );
+      watcher.observe(node);
+    };
+    if (eager || typeof IntersectionObserver === "undefined") {
+      now(value);
+    } else {
+      later(value);
+    }
     return {
       destroy() {
+        watcher?.disconnect();
         dispose?.();
       },
       update(next: string) {
-        dispose?.();
+        if (watcher) {
-        dispose = host.markdown?.(node, next);
+          later(next);
+        } else {
+          now(next);
+        }
       },
     };
   }

ui/src/lib/rail/rail.svelte

@@ -132,6 +132,7 @@
   }
 
   const refresh = () => index.load().catch(() => undefined);
+  const warm = (id: string) => index.prefetch([id]);
 
   // What the agent did on its own that day: digests and scheduled runs.
   function quietSummary(group: DayGroup): string {
@@ -208,6 +209,7 @@
                 {now}
                 onChanged={refresh}
                 onclick={onOpen}
+                onWarm={warm}
                 {row}
               />
             {/each}
@@ -283,6 +285,7 @@
                   {now}
                   onChanged={refresh}
                   onclick={onOpen}
+                  onWarm={warm}
                   row={group.master}
                 />
               {/if}
@@ -295,6 +298,7 @@
                   {now}
                   onChanged={refresh}
                   onclick={onOpen}
+                  onWarm={warm}
                   {row}
                 />
               {/each}
@@ -307,6 +311,7 @@
                   {now}
                   onChanged={refresh}
                   onclick={onOpen}
+                  onWarm={warm}
                   {row}
                 />
               {/each}
@@ -319,6 +324,7 @@
                   {now}
                   onChanged={refresh}
                   onclick={onOpen}
+                  onWarm={warm}
                   {row}
                 />
               {/each}
@@ -347,6 +353,7 @@
                       {now}
                       onChanged={refresh}
                       onclick={onOpen}
+                      onWarm={warm}
                       {row}
                     />
                   {/each}

ui/src/lib/shell/board.svelte

@@ -79,6 +79,7 @@
   const href = (id: string) => `${base}/conversations/${id}`;
   const GRAPH_FILES = 24;
   const refresh = () => gateway.load().catch(() => undefined);
+  const warm = (id: string) => gateway.prefetch([id]);
 
   async function loadVault(masterId: string) {
     const { client } = session;
@@ -414,6 +415,7 @@
             {now}
             onChanged={refresh}
             onclick={open}
+            onWarm={warm}
             open={Boolean(info?.question)}
             {row}
             state="waiting"
@@ -449,6 +451,7 @@
             {now}
             onChanged={refresh}
             onclick={open}
+            onWarm={warm}
             {row}
             state="running"
           />
@@ -532,6 +535,7 @@
             {now}
             onChanged={refresh}
             onclick={open}
+            onWarm={warm}
             {row}
             state="quiet"
           />

ui/src/lib/shell/strip.svelte

@@ -21,6 +21,7 @@
     onclick,
     client = null,
     onChanged,
+    onWarm,
   }: {
     row: ConversationSummary;
     href?: string;
@@ -36,6 +37,8 @@
     // press) with the conversation's actions.
     client?: ApiClient | null;
     onChanged?: () => void;
+    // The pointer is over the strip: a good moment to fetch what it opens.
+    onWarm?: (id: string) => void;
   } = $props();
 
   const TITLE_MAX = 72;
@@ -82,6 +85,7 @@
     data-state={state}
     href={href ?? undefined}
     onclick={activate}
+    onpointerenter={onWarm ? () => onWarm(row.id) : undefined}
     role={href ? undefined : "button"}
     tabindex="0"
     this={tag}