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

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}`;