Bug: "Load earlier" on a long session throws "system-message withdrew materialized target chat" and breaks Trajectory

[sugar-zero]

Summary

In a long-running session, using Load earlier on the conversation history can crash the client-side conversation assembler:

Error: conversation Definition "system-message" withdrew materialized target "chat"; return the same key with hidden visibility instead

After this, the session's chat UI stops applying history updates and the Trajectory tab no longer opens until the window is reloaded.

Environment

• DSH 0.2.1-rc.3 (desktop)
• macOS 27.0.1 (build 26A434)
• Reproduced in a fresh Chromium/Playwright client authenticated to the same local DSH web server, so this is not specific to the Electron window.
• Session scale: ~122 turns, ~13k durable events, multiple compactions, and seeded/inherited history.

Steps to reproduce

1. Open a large session (in my case ~13k durable events).
2. Scroll to the top of the conversation and click 加载更早 / Load earlier.
3. The first older page loads successfully.
4. Click Load earlier again.
5. Observe the console error above. The button appears to do nothing afterwards, history no longer prepends, and the 轨迹 / Trajectory tab no longer opens.

Expected behavior

Older history should prepend without errors, and chat plus Trajectory should remain usable.

Actual behavior

Both calls to POST /api/session/page return 200 OK; the failure is client-side.

Relevant console output:

[session-controller] event feed subscriber failed:
Error: conversation Definition "system-message" withdrew materialized target "chat";
return the same key with hidden visibility instead
    at ConversationNodeAssembler.buildTargetUpserts (.../dsh-client-ui-conversation/client.js:3204)
    at ConversationNodeAssembler.flush (...)

Once this happens, subsequent history loads are silent no-ops. The Trajectory view is also unresponsive because the event feed subscriber has failed.

Likely root cause

In @deepseek-ai/dsh-client-ui-conversation, buildTargetUpserts currently treats a materialized target becoming null as an invariant violation:

const previous = context.current.get(target) ?? null;
const node = this.buildNode(context, target);

if (node === null && previous !== null) {
  throw new Error(
    `conversation Definition "${context.kind}" withdrew materialized target "${target}"...`
  );
}

The Chat system-message definition can return null from buildViewNode when:

• the start event is not a system/message, or
• system/message state text is empty, or
• the event is not an append (e.g. system/message replacement handling), or
• the state is not materialized in the current window.

When an older page is prepended, a previously materialized system-message/chat target can transition to null, which triggers the throw. My session contains system/message events together with paginated request headers and seeded history around this boundary; other long sessions without that event sequence do not reproduce.

Possible fix directions

1. In the system-message definition, when the node is conceptually hidden, return the same key with hidden visibility instead of null (as the error message suggests).
2. Or make the conversation assembler tolerate withdrawal during historical prepend/rebuild, instead of treating it as an invariant violation.
3. Add a regression test that prepends a page containing system/message events after an initial snapshot.

Workaround

• Reloading the window clears the broken state.
• Opening Trajectory before using Load earlier works.
• Starting a new session avoids the issue, but does not let me inspect the old history.

I can provide a sanitized/synthetic session fixture or a smaller event sequence if that would help reproduce it.

Thanks!