fix(tui): preserve shared prompts when scrollback is full (#115165)

src/tui/components/chat-log.test.ts

@@ -387,6 +387,38 @@ describe("ChatLog", () => {
     expect(normalizeTestText(chatLog.render(120).join("\n"))).toContain("Still streaming.");
   });
 
+  it("preserves a delayed shared prompt and its active reply when scrollback is full", () => {
+    const chatLog = new ChatLog(20);
+    chatLog.updateAssistant("Already streaming.", "shared-run");
+    for (let index = 0; index < 19; index += 1) {
+      chatLog.addSystem(`Older notice ${index}.`);
+    }
+
+    chatLog.addLiveUser("Sent from the other client.", {
+      messageId: "shared-overflow-user",
+      runId: "shared-run",
+    });
+    chatLog.addLiveUser("Sent from the other client.", {
+      messageId: "shared-overflow-user",
+      runId: "shared-run",
+    });
+
+    const rendered = normalizeTestText(chatLog.render(120).join("\n"));
+    expect(chatLog.children).toHaveLength(20);
+    expect(rendered).toContain("Sent from the other client.");
+    expect(rendered).toContain("Already streaming.");
+    expect(rendered).not.toContain("Older notice 0.");
+    expect(rendered.match(/Sent from the other client\./g)).toHaveLength(1);
+    expect(rendered.indexOf("Sent from the other client.")).toBeLessThan(
+      rendered.indexOf("Already streaming."),
+    );
+
+    chatLog.updateAssistant("Still streaming after overflow.", "shared-run");
+    expect(normalizeTestText(chatLog.render(120).join("\n"))).toContain(
+      "Still streaming after overflow.",
+    );
+  });
+
   it("deduplicates authoritative user events and adopts the matching pending prompt", () => {
     const chatLog = new ChatLog(40);
     chatLog.addPendingUser("shared-run", "Persisted prompt.");

src/tui/components/chat-log.ts

@@ -87,9 +87,11 @@ export class ChatLog extends Container {
     }
   }
 
-  private pruneOverflow() {
+  private pruneOverflow(protectedComponents?: ReadonlySet<Component>) {
     while (this.children.length > this.maxComponents) {
-      const oldest = this.children[0];
+      const oldest = protectedComponents
+        ? this.children.find((component) => !protectedComponents.has(component))
+        : this.children[0];
       if (!oldest) {
         return;
       }
@@ -231,12 +233,13 @@ export class ChatLog extends Container {
       frozen?.values().next().value ??
       (options.runId ? this.streamingRuns.get(options.runId) : undefined);
     const assistantIndex = assistant ? this.children.indexOf(assistant) : -1;
-    if (assistantIndex >= 0) {
+    if (assistant && assistantIndex >= 0) {
       // Transcript broadcasts can trail the first delta; insert their prompt
-      // before the existing reply without resetting live stream or tool state.
+      // before the existing reply. Preserve both when full scrollback evicts
+      // older components so the newly recovered prompt cannot disappear.
       this.repeatableSystemMessage = null;
       this.children.splice(assistantIndex, 0, component);
-      this.pruneOverflow();
+      this.pruneOverflow(new Set([component, assistant]));
       return component;
     }
     this.appendNonSystem(component);