Keep focused agent timelines live and catch up instantly (#2196)

* fix(app): keep focused agent timelines live

Window focus was conflated with app visibility, so switching OS windows could remove the selected timeline subscription. Separate those signals, grace every visibility-driven removal, and cover retained history until authoritative catch-up completes.

* fix(app): keep timeline catch-up failures non-blocking

* fix(app): surface timeline catch-up failures

* fix(app): preserve file preview refocus

* fix(app): preserve optimistic catch-up flow

packages/app/src/hooks/use-agent-screen-state-machine.test.ts

@@ -63,6 +63,7 @@ function createBaseInput(): AgentScreenMachineInput {
     isArchivingCurrentAgent: false,
     isHistorySyncing: false,
     needsAuthoritativeSync: false,
+    visibilityCatchUpStatus: "ready",
     hasHydratedHistoryBefore: false,
   };
 }
@@ -189,6 +190,43 @@ describe("deriveAgentScreenViewState", () => {
     expect(sync.ui).toBe("silent");
   });
 
+  it("covers already-hydrated history while a newly visible agent catches up", () => {
+    const memory = createBaseMemory({
+      hasRenderedReady: true,
+      lastReadyAgent: createAgent("agent-1"),
+    });
+    const input: AgentScreenMachineInput = {
+      ...createBaseInput(),
+      agent: createAgent("agent-1"),
+      hasHydratedHistoryBefore: true,
+      visibilityCatchUpStatus: "pending",
+    };
+
+    const result = deriveAgentScreenViewState({ input, memory });
+    const ready = expectReadyState(result.state);
+    const sync = expectCatchingUpSync(ready);
+
+    expect(sync.ui).toBe("overlay");
+  });
+
+  it("keeps hydrated history readable after a visibility catch-up error", () => {
+    const memory = createBaseMemory({
+      hasRenderedReady: true,
+      lastReadyAgent: createAgent("agent-1"),
+    });
+    const input: AgentScreenMachineInput = {
+      ...createBaseInput(),
+      agent: createAgent("agent-1"),
+      hasHydratedHistoryBefore: true,
+      visibilityCatchUpStatus: "error",
+    };
+
+    const result = deriveAgentScreenViewState({ input, memory });
+    const ready = expectReadyState(result.state);
+
+    expectSyncErrorSync(ready);
+  });
+
   it("keeps sync errors non-blocking once the screen was ready", () => {
     const memory = createBaseMemory({
       hasRenderedReady: true,
@@ -328,7 +366,7 @@ describe("deriveAgentScreenViewState", () => {
     expect(result.memory.lastReadyAgent).toBeNull();
   });
 
-  it("still allows optimistic create flow to render before authoritative history arrives", () => {
+  it("keeps optimistic create non-blocking while timeline and authoritative history catch up", () => {
     const memory = createBaseMemory();
     const input: AgentScreenMachineInput = {
       ...createBaseInput(),
@@ -336,6 +374,7 @@ describe("deriveAgentScreenViewState", () => {
       continuity: { kind: "optimistic-create", agent: createAgent("agent-1") },
       needsAuthoritativeSync: true,
       isHistorySyncing: true,
+      visibilityCatchUpStatus: "pending",
       hasHydratedHistoryBefore: false,
     };