fix(ui): incorporate PR #45743 improvements for #45707

Enhancements from PR #45743:
1. Check streamSegments and live stream for session activity
2. Add min-height: 0 to .chat-main for flex layout fix
3. Expand test coverage to 6 cases (streaming scenarios)

Retained improvements:
- Three-layer defense (z-index, pointer-events)
- Comprehensive PR description
- Defense-in-depth against regressions

Fixes #45707

ui/src/styles/chat/sidebar.css

@@ -9,6 +9,7 @@
 
 .chat-main {
   min-width: 400px;
+  min-height: 0; /* Prevent flex overflow that can hide input area */
   display: flex;
   flex-direction: column;
   overflow: hidden;

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

@@ -1,43 +1,105 @@
 /**
  * Tests for #45707 - WebChat empty state overlay blocking input
+ *
+ * This test ensures that sessions with tool-call messages (heartbeat/cron),
+ * streaming content, or live streams are not treated as empty, preventing
+ * the welcome overlay from blocking the input box.
  */
 
 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);
+    const messages: unknown[] = [];
+    const toolMessages: unknown[] = [{ role: "tool", content: "Heartbeat", timestamp: Date.now() }];
+    const streamSegments: Array<{ text: string; ts: number }> = [];
+    const stream: string | null = null;
+
+    const hasSessionActivity =
+      messages.length > 0 ||
+      toolMessages.length > 0 ||
+      streamSegments.some((segment) => segment.text.trim()) ||
+      stream !== null;
+
+    expect(hasSessionActivity).toBe(true);
   });
 
   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);
+    const messages: unknown[] = [];
+    const toolMessages: unknown[] = [];
+    const streamSegments: Array<{ text: string; ts: number }> = [];
+    const stream: string | null = null;
+
+    const hasSessionActivity =
+      messages.length > 0 ||
+      toolMessages.length > 0 ||
+      streamSegments.some((segment) => segment.text.trim()) ||
+      stream !== null;
+
+    expect(hasSessionActivity).toBe(false);
   });
 
   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);
+    const messages: unknown[] = [{ role: "user", content: "Hello", timestamp: Date.now() }];
+    const toolMessages: unknown[] = [];
+    const streamSegments: Array<{ text: string; ts: number }> = [];
+    const stream: string | null = null;
+
+    const hasSessionActivity =
+      messages.length > 0 ||
+      toolMessages.length > 0 ||
+      streamSegments.some((segment) => segment.text.trim()) ||
+      stream !== null;
+
+    expect(hasSessionActivity).toBe(true);
   });
 
-  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);
+  it("should not treat session with streaming content as empty", () => {
+    const messages: unknown[] = [];
+    const toolMessages: unknown[] = [];
+    const streamSegments: Array<{ text: string; ts: number }> = [
+      { text: "Streaming response...", ts: Date.now() },
+    ];
+    const stream: string | null = null;
+
+    const hasSessionActivity =
+      messages.length > 0 ||
+      toolMessages.length > 0 ||
+      streamSegments.some((segment) => segment.text.trim()) ||
+      stream !== null;
+
+    expect(hasSessionActivity).toBe(true);
+  });
+
+  it("should not treat session with live stream as empty", () => {
+    const messages: unknown[] = [];
+    const toolMessages: unknown[] = [];
+    const streamSegments: Array<{ text: string; ts: number }> = [];
+    const stream: string | null = "Live streaming...";
+
+    const hasSessionActivity =
+      messages.length > 0 ||
+      toolMessages.length > 0 ||
+      streamSegments.some((segment) => segment.text.trim()) ||
+      stream !== null;
+
+    expect(hasSessionActivity).toBe(true);
+  });
+
+  it("should handle mixed messages and streaming", () => {
+    const messages: unknown[] = [{ role: "user", content: "Check", timestamp: Date.now() }];
+    const toolMessages: unknown[] = [{ role: "tool", content: "Cron", timestamp: Date.now() }];
+    const streamSegments: Array<{ text: string; ts: number }> = [
+      { text: "Thinking...", ts: Date.now() },
+    ];
+    const stream: string | null = null;
+
+    const hasSessionActivity =
+      messages.length > 0 ||
+      toolMessages.length > 0 ||
+      streamSegments.some((segment) => segment.text.trim()) ||
+      stream !== null;
+
+    expect(hasSessionActivity).toBe(true);
   });
 });

ui/src/ui/views/chat.ts

@@ -850,12 +850,16 @@ export function renderChat(props: ChatProps) {
   };
 
   const chatItems = buildChatItems(props);
-  // 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;
+  // Fix #45707: Check for any session activity, not just renderable messages
+  // Includes tool-call messages (heartbeat/cron), streaming content, and live streams
+  const hasSessionActivity =
+    (Array.isArray(props.messages) && props.messages.length > 0) ||
+    (Array.isArray(props.toolMessages) && props.toolMessages.length > 0) ||
+    (Array.isArray(props.streamSegments) &&
+      props.streamSegments.some((segment) => segment.text.trim())) ||
+    props.stream !== null;
+  const showWelcomeState = !hasSessionActivity && !props.loading && !vs.searchOpen;
+  const showEmptySearch = chatItems.length === 0 && !props.loading && vs.searchOpen;
 
   const thread = html`
     <div
@@ -898,9 +902,9 @@ export function renderChat(props: ChatProps) {
             `
           : nothing
       }
-      ${isEmpty && !vs.searchOpen ? renderWelcomeState(props) : nothing}
+      ${showWelcomeState ? renderWelcomeState(props) : nothing}
       ${
-        isEmpty && vs.searchOpen
+        showEmptySearch
           ? html`
               <div class="agent-chat__empty">No matching messages</div>
             `