Improve built-in terminal reliability and remote replay performance (get-bb#1076)

Closes get-bb#1075.

## Summary

- batch raw PTY output into ordered chunks and preserve arbitrary
UTF-8/binary bytes end to end
- add bounded browser and daemon websocket backpressure, bad-socket
isolation, heartbeat/reconnect, input queuing, and sequence
deduplication
- reconnect from the last received sequence and bound initial browser
replay to the latest 512 KiB instead of replaying the full 4 MiB
scrollback on every open
- serialize terminal restart, open the replacement before closing the
old PTY, and wait for daemon exit acknowledgement so final output
precedes exit
- keep threadless terminal caches current, make latest-attached clients
own resize, and restore ownership on failed attach/detach
- enable xterm Unicode 11 width rules and Nerd Font fallbacks
- bump the host-daemon protocol from 75 to 76 for byte-preserving input
and required bounded-replay fields

## Measurements

### Sustained output

A local 16 MiB output probe improved from about 6.5 MiB/s to 53–63
MiB/s. Websocket output messages dropped from 16,386 to 258–480
depending on PTY chunking.

Input remained responsive during a 16 MiB remote flood. Over the
Tailscale path used for QA, the full burst completed without disconnect
while a concurrent input echo returned in 101 ms.

### Reopening a populated remote terminal

Measured through an SSH tunnel to a patched VPS:

| | Before | After |
|---|---:|---:|
| Time to final replayed line | 2,210 ms | 556 ms |
| Replay bytes | 4,192,407 | 461,853 |
| Websocket output messages | 1,025 | 15 |

A subsequent reconnect requested the previous `nextSeq` and replayed
only 319 bytes rather than the retained history.

### Input integrity

A 200,009-byte payload containing emoji and combining characters crossed
multiple 64 KiB request boundaries on the VPS with an exact SHA-256
match. CLI `--stdin` also has coverage for invalid UTF-8 bytes.

## Validation

On current `main`:

- `pnpm exec turbo run typecheck` for app, CLI, host daemon, host-daemon
contract, server, SDK, server contract, and templates
- 191 focused tests across app terminal UI/transport/controller, CLI
terminal commands, daemon connection/PTY manager, server hub/routes, and
both contracts
- production builds for app, CLI, server, and host daemon
- patched local and VPS live stacks, including latency,
flood/backpressure, atomic restart, large paste, replay truncation,
reconnect, and multi-client resize ownership

---------

Co-authored-by: Michael Yong <wrong92@gmail.com>

Author: 2 people

apps/app/package.json

@@ -76,6 +76,7 @@
     "@tiptap/starter-kit": "^3.26.0",
     "@tiptap/suggestion": "^3.26.0",
     "@xterm/addon-fit": "0.12.0-beta.292",
+    "@xterm/addon-unicode11": "^0.9.0",
     "@xterm/addon-web-links": "0.13.0-beta.292",
     "@xterm/addon-webgl": "0.20.0-beta.291",
     "@xterm/xterm": "6.1.0-beta.292",
@@ -129,10 +130,10 @@
     "@types/node": "^22.0.0",
     "@types/react": "^19.0.0",
     "@types/react-dom": "^19.0.0",
-    "bb-plugin-automations": "workspace:*",
     "@typescript-eslint/parser": "^8.63.0",
     "@vitejs/plugin-react": "^6.0.1",
     "babel-plugin-react-compiler": "^1.0.0",
+    "bb-plugin-automations": "workspace:*",
     "eslint": "^9.39.3",
     "eslint-plugin-react-hooks": "^7.0.1",
     "mdast-util-to-hast": "^13.2.1",

apps/app/src/components/thread/terminal/ThreadTerminalContent.test.tsx

@@ -36,6 +36,7 @@ function controller(isPanelOpen: boolean): ThreadTerminalController {
     canCreateTerminal: true,
     closingTerminalId: null,
     emptyTerminalMessage: "No terminals",
+    handleActiveTerminalSessionChange: () => undefined,
     handleActiveTerminalTitleChange: () => undefined,
     handleActiveTerminalUserInput: () => undefined,
     handleClosePanel: () => undefined,

apps/app/src/components/thread/terminal/ThreadTerminalContent.tsx

@@ -129,6 +129,7 @@ export function ThreadTerminalContent({
       isPanelOpen={controller.isPanelOpen}
       onOpenLink={onOpenLink}
       onSelectionAddToChat={onSelectionAddToChat}
+      onSessionChange={controller.handleActiveTerminalSessionChange}
       onTitleChange={controller.handleActiveTerminalTitleChange}
       onUserInput={controller.handleActiveTerminalUserInput}
       session={controller.activeSession}

apps/app/src/components/thread/terminal/ThreadTerminalPanel.stories.tsx

@@ -76,6 +76,8 @@ function noopAction(): void {}
 
 function noopTerminalIdAction(_terminalId: string): void {}
 
+function noopSessionChange(_session: TerminalSession): void {}
+
 function noopTitleChange(_title: string): void {}
 
 function makeController({
@@ -98,6 +100,7 @@ function makeController({
     canCreateTerminal,
     closingTerminalId,
     emptyTerminalMessage,
+    handleActiveTerminalSessionChange: noopSessionChange,
     handleActiveTerminalTitleChange: noopTitleChange,
     handleActiveTerminalUserInput: noopAction,
     handleClosePanel: noopAction,
@@ -237,10 +240,7 @@ function RunningTerminalPreview() {
 export function Overview() {
   return (
     <StoryCard labelWidth="190px">
-      <StoryRow
-        label="disconnected"
-        hint="Replacement available."
-      >
+      <StoryRow label="disconnected" hint="Replacement available.">
         <TerminalContentStage controller={disconnectedController} />
       </StoryRow>
       <StoryRow
@@ -252,7 +252,10 @@ export function Overview() {
       <StoryRow label="starting" hint="Session exists but is not running yet.">
         <TerminalContentStage controller={startingController} />
       </StoryRow>
-      <StoryRow label="exited" hint="Terminal has ended and cannot accept input.">
+      <StoryRow
+        label="exited"
+        hint="Terminal has ended and cannot accept input."
+      >
         <TerminalContentStage controller={exitedController} />
       </StoryRow>
       <StoryRow label="empty" hint="Right panel tab with no visible sessions.">

apps/app/src/components/thread/terminal/ThreadTerminalView.test.ts

@@ -1,10 +1,110 @@
+import { TERMINAL_DATA_MAX_BYTES } from "@bb/domain";
+import { Terminal } from "@xterm/xterm";
 import { describe, expect, it, vi } from "vitest";
 import {
   buildTerminalThemeFromCssColors,
+  decodeTerminalOutputBytes,
+  encodeTerminalInputChunks,
+  forwardTerminalData,
   loadOptionalTerminalWebglAddon,
   loadTerminalWebglRenderer,
+  TERMINAL_ALLOW_PROPOSED_API,
+  TERMINAL_FONT_FAMILY,
+  TERMINAL_UNICODE_VERSION,
+  writeTerminalOutput,
 } from "./ThreadTerminalView";
 
+describe("terminal output encoding", () => {
+  it("splits large paste input at the wire limit without losing UTF-8 bytes", () => {
+    const input = `${"a".repeat(TERMINAL_DATA_MAX_BYTES - 1)}🙂tail`;
+    const chunks = encodeTerminalInputChunks(input);
+    const decoded = Buffer.concat(
+      chunks.map((chunk) => Buffer.from(chunk, "base64")),
+    );
+
+    expect(chunks).toHaveLength(2);
+    expect(
+      chunks.every(
+        (chunk) =>
+          Buffer.from(chunk, "base64").byteLength <= TERMINAL_DATA_MAX_BYTES,
+      ),
+    ).toBe(true);
+    expect(decoded.toString("utf8")).toBe(input);
+  });
+
+  it("keeps UTF-8 bytes intact when a glyph spans output chunks", () => {
+    const encoded = new TextEncoder().encode("🙂");
+    const first = decodeTerminalOutputBytes(
+      Buffer.from(encoded.subarray(0, 2)).toString("base64"),
+    );
+    const second = decodeTerminalOutputBytes(
+      Buffer.from(encoded.subarray(2)).toString("base64"),
+    );
+    const decoder = new TextDecoder();
+
+    expect(
+      decoder.decode(first, { stream: true }) + decoder.decode(second),
+    ).toBe("🙂");
+  });
+
+  it("does not send terminal protocol replies generated by replayed output", async () => {
+    const terminal = new Terminal({ cols: 80, rows: 24 });
+    const onInput = vi.fn<(dataBase64: string) => void>();
+    const onUserInput = vi.fn();
+    const replayWriteState = { suppressedWriteCount: 0 };
+    terminal.onData((data) => {
+      forwardTerminalData({
+        data,
+        onInput,
+        onUserInput,
+        replayWriteState,
+        sessionStatus: "running",
+      });
+    });
+
+    writeTerminalOutput({
+      data: "\u001b[6n",
+      isReplay: true,
+      replayWriteState,
+      terminal,
+    });
+    await new Promise<void>((resolve) => terminal.write("", resolve));
+
+    expect(onInput).not.toHaveBeenCalled();
+    expect(onUserInput).not.toHaveBeenCalled();
+
+    writeTerminalOutput({
+      data: "\u001b[6n",
+      isReplay: false,
+      replayWriteState,
+      terminal,
+    });
+    await new Promise<void>((resolve) => terminal.write("", resolve));
+
+    expect(onInput).toHaveBeenCalledOnce();
+    const encodedReply = onInput.mock.calls[0]?.[0];
+    if (encodedReply === undefined) {
+      throw new Error("Expected xterm to emit a cursor-position reply");
+    }
+    expect(Buffer.from(encodedReply, "base64").toString("utf8")).toBe(
+      "\u001b[1;1R",
+    );
+    expect(onUserInput).toHaveBeenCalledOnce();
+    expect(replayWriteState.suppressedWriteCount).toBe(0);
+    terminal.dispose();
+  });
+
+  it("enables the proposed xterm API required by the Unicode addon", () => {
+    expect(TERMINAL_ALLOW_PROPOSED_API).toBe(true);
+    expect(TERMINAL_UNICODE_VERSION).toBe("11");
+  });
+
+  it("prefers installed Nerd Font families before system monospace fallbacks", () => {
+    expect(TERMINAL_FONT_FAMILY).toContain("Nerd Font");
+    expect(TERMINAL_FONT_FAMILY).toContain("ui-monospace");
+  });
+});
+
 describe("buildTerminalThemeFromCssColors", () => {
   it("paints the terminal canvas and cursor cutout with the sidebar surface", () => {
     const get = vi.fn((name: string) => name);