diff --git a/docs/plans/2026-08-14-terminal-open-when-sized-design.md b/docs/plans/2026-08-14-terminal-open-when-sized-design.md
new file mode 100644
index 0000000..cc36f31
--- /dev/null
+++ b/docs/plans/2026-08-14-terminal-open-when-sized-design.md
@@ -0,0 +1,90 @@
+# 终端延迟初始化（零尺寸容器 xterm open 崩溃修复）设计
+
+**日期**：2026-08-14
+**状态**：已实施（本文档含实施偏差记录）
+**目标版本**：v0.11.x（issue #25 修复）
+
+## 1. 目标
+
+修复 [issue #25](https://github.com/omdsh-dev/DSH-better-sidebar/issues/25)：WKWebView 下点击「展开底部面板」后底部面板内容空白、无终端，控制台报 `Cannot read properties of undefined (reading 'dimensions')` 与 `(reading 'handleResize')`。
+
+根因：`TerminalView` 的挂载 effect 在容器可能为零尺寸时**同步**执行 `term.open(host); fit.fit()`。xterm 5.3.0 在零尺寸容器里 open 时渲染器创建失败，`RenderService._renderer.value` 保持 undefined，随后 Viewport 的 `_innerRefresh` 读 `_renderService.dimensions`（getter 即 `_renderer.value.dimensions`）崩溃。Chrome（Blink）对零尺寸布局更宽容所以不崩；WKWebView 稳定复现。
+
+修复：把 `term.open(host)` + `fit.fit()` 推迟到容器有实际尺寸之后再执行（xterm 在隐藏/零尺寸容器初始化的标准修法），open 恰好执行一次。
+
+## 2. 非目标
+
+- 不换 xterm 渲染器（打包产物只有 DomRenderer 是现状，不是本次根因；`rendererType` 保持默认）。
+- 不改 pty / WS / 终端连接生命周期：推迟 open 不推迟连接、不重启 shell、不丢 transcript。
+- 不改 host 半与 chunk 打包结构（新模块随 terminal chunk 打入 `lib/client-terminal.js`）。
+- 不 bump 版本号（release 流程负责）。
+
+## 3. 现状回顾
+
+- `src/client/TerminalView.tsx` 第 99–101 行同步 `term.open(host); fit.fit()`。
+- 零尺寸挂载路径真实存在且不止一条：
+  - 底部面板首次展开自动开终端（`Sidebar.tsx` 的 `bottomOpenedOnce` effect）在面板 slide-in transition 的同一次 commit 挂载 TerminalView，WKWebView 下此刻 host 高度为 0；
+  - 所有 tab 保持挂载（`split-pane.tsx`：非激活 tab 用 `.paneTabHidden { display: none }` 隐藏），面板折叠时保持挂载（`.panelHidden`）——任何 `display:none` / 零尺寸祖先都会踩同一坑。
+- 前提已验证（xterm 5.3.0 与 @xterm/addon-fit 0.10.0 源码）：
+  - `FitAddon.fit()` 在 open 之前是安全 no-op（`proposeDimensions` 先查 `this._terminal.element`，undefined 即返回）。
+  - `WriteBuffer.write()` 不依赖 open 状态：数据解析进 buffer，open 后渲染——推迟 open 不丢初始 transcript。
+  - `TerminalView` 已有的 ResizeObserver / 字体订阅里的 `fit.fit()` 在 open 前 no-op、open 后照常 re-fit，无需改动。
+
+## 4. 设计
+
+### 4.1 新模块 `src/client/open-when-sized.ts`（纯逻辑，可单测）
+
+```ts
+export function openWhenSized(
+  host: HTMLElement,
+  open: () => void,
+  raf: (cb: FrameRequestCallback) => number = requestAnimationFrame,
+  caf: (id: number) => void = cancelAnimationFrame,
+): () => void
+```
+
+- 单条代码路径：`raf(step)` → step 检查 `host.isConnected`（false 则停止，防卸载后空转）→ `clientWidth > 0 && clientHeight > 0` 则调 `open()` 并停止；否则再排一帧。
+- 返回 cancel：`caf(frame)` 并置空（幂等），cleanup 调用。
+- helper 不 try/catch（保持纯净），异常由调用方处理。
+
+### 4.2 消费点 `src/client/TerminalView.tsx`
+
+- 移除 effect 顶部的 `term.open(host); fit.fit()`（保留 `term.loadAddon(fit)`）。
+- 在 effect 尾部（`sendResize` / `connect` 定义之后、`connect()` 调用之前）注册：
+
+```ts
+const cancelOpen = openWhenSized(host, () => {
+  try {
+    term.open(host)
+    fit.fit()
+    sendResize()
+  } catch (error) {
+    console.error('[dsh-better-sidebar] xterm open failed:', error)
+  }
+})
+```
+
+- 回调里补 `sendResize()`：推迟路径下 socket 可能已先连上（`connect()` 在 effect 尾部同步调用），open+fit 后必须把真实 cols/rows 发出去，否则 pty 停留在默认 80x24 直到下次 resize；已尺寸快速路径下 socket 尚未 open，`sendResize` 内部守卫（`readyState === OPEN`）自然 no-op，无行为变化。
+- cleanup 里在 `term.dispose()` 前加 `cancelOpen()`。
+- 其余（connect / observer / fontSub / schemeSub / onData / 主题）不动。
+
+## 5. 测试
+
+`tests/open-when-sized.spec.ts`（jsdom + 注入手动步进的假 raf/caf；`Object.defineProperty` 桩 `clientWidth/clientHeight`；`appendChild/remove` 控制 `isConnected`）：
+
+1. 已尺寸 → tick 一次后 open 恰好一次，之后不再排帧；
+2. 零尺寸 → 多次 tick 不 open，设尺寸后下一次 tick open 一次，后续 tick 不重复；
+3. 只有一维缺失 → 补齐后 open；
+4. cancel 后永不 open、无残留帧、cancel 幂等；
+5. host 脱离文档 → 轮询停止、不 open；
+6. open 抛错 → 异常向调用方传播（调用方负责 try/catch），不重复调用。
+
+## 6. 限制与取舍
+
+- rAF 轮询在 host 长期隐藏（`display:none` 祖先）期间持续排帧，每帧只读两个属性，开销可忽略；open 后立即停止。用 ResizeObserver 触发也可行，但轮询是 issue 中建议的标准修法，且不依赖 observer 对 transition 中间态的回调时序。
+- 已尺寸快速路径比原行为晚一帧（rAF 回调时机），无感知差异。
+- 若容器永远零尺寸（异常布局），终端永不 open、不崩溃——对不可见内容是可接受的降级。
+
+## 7. 实施偏差记录
+
+无（按设计实施）。
diff --git a/src/client/TerminalView.tsx b/src/client/TerminalView.tsx
index 83a34b5..d4ec208 100644
--- a/src/client/TerminalView.tsx
+++ b/src/client/TerminalView.tsx
@@ -24,6 +24,7 @@ import { Terminal, type ITheme } from 'xterm'
 import { FitAddon } from '@xterm/addon-fit'
 import 'xterm/css/xterm.css'
 import { t } from './locales.ts'
+import { openWhenSized } from './open-when-sized.ts'
 import type { SessionScope } from './api.ts'
 import { agentUuidOf, isAgentTabId, type SidebarStore } from './state.ts'
 import { isDarkScheme, subscribeColorScheme, tokenValue } from './theme.ts'
@@ -97,8 +98,6 @@ export function TerminalView(props: { scope: SessionScope; tabId: string; store:
     })
     const fit = new FitAddon()
     term.loadAddon(fit)
-    term.open(host)
-    fit.fit()
     // Re-theme in place when the app's scheme flips (tokens + palette).
     const applyTheme = (): void => {
       term.options.theme = xtermTheme()
@@ -208,9 +207,30 @@ export function TerminalView(props: { scope: SessionScope; tabId: string; store:
       }
     })
 
+    // The terminal must not be opened in a zero-size container: xterm's
+    // renderer creation fails there and the next Viewport refresh crashes
+    // reading `.dimensions` off the undefined renderer (blank terminal on
+    // WKWebView when the bottom panel's expand slide leaves the host at
+    // height 0; any display:none-hidden ancestor does the same). Defer
+    // open+fit until the host has a real size — writes arriving meanwhile
+    // are buffered by xterm's WriteBuffer and render once open, and
+    // FitAddon.fit() is a safe no-op before open. sendResize() here covers
+    // the deferred path where the socket may already be open with the
+    // default 80x24 dims.
+    const cancelOpen = openWhenSized(host, () => {
+      try {
+        term.open(host)
+        fit.fit()
+        sendResize()
+      } catch (error) {
+        console.error('[dsh-better-sidebar] xterm open failed:', error)
+      }
+    })
+
     connect()
     return () => {
       closed = true
+      cancelOpen()
       window.clearTimeout(retry)
       observer.disconnect()
       fontSub()
diff --git a/src/client/open-when-sized.ts b/src/client/open-when-sized.ts
new file mode 100644
index 0000000..481b554
--- /dev/null
+++ b/src/client/open-when-sized.ts
@@ -0,0 +1,43 @@
+/**
+ * Deferred one-shot open for hosts that may not have a real size yet.
+ *
+ * xterm's `Terminal.open()` must not run in a zero-size container: the
+ * renderer creation fails there (the DomRenderer is built from the host's
+ * dimensions), leaving the render service's renderer `undefined`, and the
+ * next Viewport refresh crashes reading `.dimensions` off it. WebKit-based
+ * hosts (WKWebView) reliably report zero while the bottom panel's expand
+ * slide is in flight; any `display:none`-hidden ancestor does the same.
+ *
+ * The caller's `open` callback (open + fit + resize) is invoked exactly
+ * once, on the first frame where the host reports a real size. While the
+ * host stays zero-sized the polling continues every frame; it stops when
+ * the host leaves the document (`isConnected`), so a pending open never
+ * fires after unmount. The returned cancel function drops a pending frame
+ * immediately (idempotent).
+ *
+ * `raf`/`caf` are injectable so tests can drive the polling deterministically.
+ */
+export function openWhenSized(
+  host: HTMLElement,
+  open: () => void,
+  raf: (cb: FrameRequestCallback) => number = requestAnimationFrame,
+  caf: (id: number) => void = cancelAnimationFrame,
+): () => void {
+  let frame: number | null = null
+  const step = (): void => {
+    frame = null
+    if (!host.isConnected) return
+    if (host.clientWidth > 0 && host.clientHeight > 0) {
+      open()
+      return
+    }
+    frame = raf(step)
+  }
+  frame = raf(step)
+  return () => {
+    if (frame !== null) {
+      caf(frame)
+      frame = null
+    }
+  }
+}
diff --git a/tests/open-when-sized.spec.ts b/tests/open-when-sized.spec.ts
new file mode 100644
index 0000000..2dcf7cf
--- /dev/null
+++ b/tests/open-when-sized.spec.ts
@@ -0,0 +1,138 @@
+/**
+ * openWhenSized tests: the deferred one-shot open guard for hosts that may
+ * not have a real size yet (xterm crashes when opened in a zero-size
+ * container — the WKWebView bottom-panel blank-terminal bug, issue #25).
+ * The polling must open exactly once, only once the host reports a real
+ * size, stop when the host leaves the document, and cancel cleanly.
+ */
+// @vitest-environment jsdom
+import { describe, expect, it } from 'vitest'
+import { openWhenSized } from '../src/client/open-when-sized.ts'
+
+/** A manually-stepped raf/caf pair so the polling is deterministic. */
+function makeScheduler(): {
+  raf: (cb: FrameRequestCallback) => number
+  caf: (id: number) => void
+  tick: () => void
+  pending: () => number
+} {
+  let nextId = 0
+  const frames = new Map<number, FrameRequestCallback>()
+  return {
+    raf: (cb) => { const id = ++nextId; frames.set(id, cb); return id },
+    caf: (id) => { frames.delete(id) },
+    tick: () => {
+      const cbs = [...frames.values()]
+      frames.clear()
+      for (const cb of cbs) cb(0)
+    },
+    pending: () => frames.size,
+  }
+}
+
+/** A host whose reported size can be changed between ticks. */
+function makeHost(width: number, height: number): {
+  el: HTMLElement
+  setSize: (w: number, h: number) => void
+} {
+  const el = document.createElement('div')
+  document.body.appendChild(el)
+  const setSize = (w: number, h: number): void => {
+    Object.defineProperty(el, 'clientWidth', { value: w, configurable: true })
+    Object.defineProperty(el, 'clientHeight', { value: h, configurable: true })
+  }
+  setSize(width, height)
+  return { el, setSize }
+}
+
+describe('openWhenSized', () => {
+  it('opens on the first tick when the host already has a size, then stops', () => {
+    const { raf, caf, tick, pending } = makeScheduler()
+    const host = makeHost(320, 200)
+    let opened = 0
+    const cancel = openWhenSized(host.el, () => { opened += 1 }, raf, caf)
+    tick()
+    expect(opened).toBe(1)
+    expect(pending()).toBe(0)
+    tick()
+    expect(opened).toBe(1)
+    cancel()
+  })
+
+  it('defers while the host is zero-sized and opens exactly once once sized', () => {
+    const { raf, caf, tick, pending } = makeScheduler()
+    const host = makeHost(0, 0)
+    let opened = 0
+    const cancel = openWhenSized(host.el, () => { opened += 1 }, raf, caf)
+    tick()
+    tick()
+    tick()
+    expect(opened).toBe(0)
+    expect(pending()).toBe(1)
+    host.setSize(320, 200)
+    tick()
+    expect(opened).toBe(1)
+    expect(pending()).toBe(0)
+    tick()
+    expect(opened).toBe(1)
+    cancel()
+  })
+
+  it('opens when only one dimension was missing', () => {
+    const { raf, caf, tick } = makeScheduler()
+    const host = makeHost(320, 0)
+    let opened = 0
+    const cancel = openWhenSized(host.el, () => { opened += 1 }, raf, caf)
+    tick()
+    expect(opened).toBe(0)
+    host.setSize(320, 120)
+    tick()
+    expect(opened).toBe(1)
+    cancel()
+  })
+
+  it('cancels a pending open and leaves no scheduled frames', () => {
+    const { raf, caf, tick, pending } = makeScheduler()
+    const host = makeHost(0, 0)
+    let opened = 0
+    const cancel = openWhenSized(host.el, () => { opened += 1 }, raf, caf)
+    tick()
+    cancel()
+    expect(pending()).toBe(0)
+    host.setSize(320, 200)
+    tick()
+    expect(opened).toBe(0)
+    // Idempotent: cancelling again is a no-op.
+    cancel()
+  })
+
+  it('stops polling when the host leaves the document', () => {
+    const { raf, caf, tick, pending } = makeScheduler()
+    const host = makeHost(0, 0)
+    let opened = 0
+    const cancel = openWhenSized(host.el, () => { opened += 1 }, raf, caf)
+    tick()
+    host.el.remove()
+    tick()
+    expect(opened).toBe(0)
+    expect(pending()).toBe(0)
+    // Even if the host somehow got a size after detaching, nothing opens.
+    host.setSize(320, 200)
+    tick()
+    expect(opened).toBe(0)
+    cancel()
+  })
+
+  it('does not swallow exceptions from open (the caller owns error handling)', () => {
+    const { raf, caf, tick } = makeScheduler()
+    const host = makeHost(320, 200)
+    let calls = 0
+    const cancel = openWhenSized(host.el, () => {
+      calls += 1
+      throw new Error('boom')
+    }, raf, caf)
+    expect(() => tick()).toThrow('boom')
+    expect(calls).toBe(1)
+    cancel()
+  })
+})