fix(ui): prevent empty state overlay from blocking chat input (#45707)

Problem: WebChat empty state covers input box for tool-only sessions
(heartbeat/cron), causing complete session lockout.

Three-layer defense:
1. Fix emptiness logic (check history + tools, not just renderable)
2. Force input z-index: 100 above overlay
3. Disable overlay pointer-events for click-through

Added: chat.empty-state.test.ts (4 test cases)
Fixes #45707

ui/src/styles/chat/layout.css

@@ -363,8 +363,10 @@
   box-sizing: border-box;
 }
 
+/* Fix #45707: Ensure input always accessible, even when empty state renders */
 .agent-chat__input {
   position: relative;
+  z-index: 100; /* Force above empty state overlay */
   display: flex;
   flex-direction: column;
   margin: 0 18px 14px;
@@ -903,6 +905,7 @@
 }
 
 /* Welcome state (new session) */
+/* Fix #45707: Constrain to message area, never cover input */
 .agent-chat__welcome {
   display: flex;
   flex-direction: column;
@@ -913,6 +916,14 @@
   padding: 48px 24px;
   flex: 1;
   min-height: 0;
+  /* Ensure welcome state stays below input box in stacking context */
+  z-index: 1;
+  pointer-events: none; /* Allow clicks to pass through to input area */
+}
+
+/* Re-enable pointer events on interactive elements within welcome state */
+.agent-chat__welcome > * {
+  pointer-events: auto;
 }
 
 .agent-chat__welcome-glow {

ui/src/ui/views/chat.empty-state.test.ts

@@ -0,0 +1,43 @@
+/**
+ * Tests for #45707 - WebChat empty state overlay blocking input
+ */
+
+import { describe, it, expect } from "vitest";
+
+describe("chat empty state logic (#45707)", () => {
+  it("should not treat session with tool messages as empty", () => {
+    const history: unknown[] = [];
+    const tools: unknown[] = [{ role: "tool", content: "Heartbeat", timestamp: Date.now() }];
+    const hasAnyMessages = history.length > 0 || tools.length > 0;
+    const isEmpty = !hasAnyMessages;
+    expect(hasAnyMessages).toBe(true);
+    expect(isEmpty).toBe(false);
+  });
+
+  it("should treat truly empty session as empty", () => {
+    const history: unknown[] = [];
+    const tools: unknown[] = [];
+    const hasAnyMessages = history.length > 0 || tools.length > 0;
+    const isEmpty = !hasAnyMessages;
+    expect(hasAnyMessages).toBe(false);
+    expect(isEmpty).toBe(true);
+  });
+
+  it("should not treat session with history messages as empty", () => {
+    const history: unknown[] = [{ role: "user", content: "Hello", timestamp: Date.now() }];
+    const tools: unknown[] = [];
+    const hasAnyMessages = history.length > 0 || tools.length > 0;
+    const isEmpty = !hasAnyMessages;
+    expect(hasAnyMessages).toBe(true);
+    expect(isEmpty).toBe(false);
+  });
+
+  it("should handle mixed history and tool messages", () => {
+    const history: unknown[] = [{ role: "user", content: "Check", timestamp: Date.now() }];
+    const tools: unknown[] = [{ role: "tool", content: "Cron", timestamp: Date.now() }];
+    const hasAnyMessages = history.length > 0 || tools.length > 0;
+    const isEmpty = !hasAnyMessages;
+    expect(hasAnyMessages).toBe(true);
+    expect(isEmpty).toBe(false);
+  });
+});

ui/src/ui/views/chat.ts

@@ -850,7 +850,12 @@ export function renderChat(props: ChatProps) {
   };
 
   const chatItems = buildChatItems(props);
-  const isEmpty = chatItems.length === 0 && !props.loading;
+  // Fix #45707: Check total message count, not just renderable items
+  // Sessions with tool-call messages (heartbeat/cron) should not be treated as empty
+  const history = Array.isArray(props.messages) ? props.messages : [];
+  const tools = Array.isArray(props.toolMessages) ? props.toolMessages : [];
+  const hasAnyMessages = history.length > 0 || tools.length > 0;
+  const isEmpty = !hasAnyMessages && !props.loading;
 
   const thread = html`
     <div