diff --git a/src/web/public/app.js b/src/web/public/app.js
index cd95f9da..b495fb8c 100644
--- a/src/web/public/app.js
+++ b/src/web/public/app.js
@@ -1340,6 +1340,14 @@ class CodemanApp {
     this._detachOrphanStrikes.delete(id);
     this.detachedWindows.delete(id);
     this._markDetached(id, false);
+    // Sizing comes back with the session. While the popup owned it the dashboard
+    // sent no resizes, so the PTY still holds the popup's geometry; the session
+    // this window is actually showing has to be re-sized to this window, and
+    // `force` is required because the dimensions the dashboard last sent are
+    // the ones it is about to send again.
+    if (id === this.activeSessionId) {
+      this.sendResize(id, { force: true })?.catch?.(() => {});
+    }
   }
 
   /** Defer a channel-driven redock briefly. A popup *reload* emits 'redocked'
diff --git a/src/web/public/terminal-ui.js b/src/web/public/terminal-ui.js
index 09c232cf..1c30ee80 100644
--- a/src/web/public/terminal-ui.js
+++ b/src/web/public/terminal-ui.js
@@ -935,7 +935,10 @@ Object.assign(CodemanApp.prototype, {
         // causes Ink to re-render at the new row count, garbling terminal output.
         // Local fit() still runs so xterm knows the viewport size for scrolling.
         const keyboardUp = typeof KeyboardHandler !== 'undefined' && KeyboardHandler.keyboardVisible;
-        if (this.activeSessionId && !keyboardUp) {
+        // Same yield as sendResize: never resize a PTY whose session is showing
+        // in its own window. Dragging the dashboard's border must not reshape it.
+        const detachedElsewhere = !this.isSoloWindow && this.detachedSessions?.has(this.activeSessionId);
+        if (this.activeSessionId && !keyboardUp && !detachedElsewhere) {
           const dims = this.fitAddon.proposeDimensions();
           // Enforce minimum dimensions to prevent layout issues
           const cols = dims ? Math.max(dims.cols, MIN_COLS) : MIN_COLS;
@@ -4824,6 +4827,13 @@ Object.assign(CodemanApp.prototype, {
    * @returns {Promise<boolean>} Whether dimensions changed from the last send
    */
   async sendResize(sessionId, options = {}) {
+    // One PTY cannot hold two sizes. A detached session is owned by its own
+    // window, and the dashboard's terminal is narrower than that window because
+    // the session rail takes width the popup does not have — so both sizing it
+    // makes the CLI draw frames that fit neither, which garbles the popup. The
+    // dashboard yields; the solo window sizes what it alone displays.
+    // (_maybeRefetchFullHistory already stands aside for the same reason.)
+    if (!this.isSoloWindow && this.detachedSessions?.has(sessionId)) return false;
     // Fit terminal to container before reading dimensions — ensures local
     // terminal size matches what we report to the server PTY.
     if (this.fitAddon) this.fitAddon.fit();
diff --git a/test/detached-session-pane-sizing.test.ts b/test/detached-session-pane-sizing.test.ts
new file mode 100644
index 00000000..65563691
--- /dev/null
+++ b/test/detached-session-pane-sizing.test.ts
@@ -0,0 +1,99 @@
+/**
+ * @fileoverview A detached session's pane is sized by its own window, not by
+ * the dashboard.
+ *
+ * One PTY holds one size. When a session is popped out, the dashboard keeps it
+ * active and keeps measuring it, but the dashboard's terminal is narrower than
+ * the popup because the session rail takes width the popup does not have. Both
+ * windows sizing the same pane makes the CLI draw frames that fit neither, and
+ * the popup shows the result as a garbled frame.
+ *
+ * `sendResize` therefore returns early for a session this window has marked
+ * detached, and the debounced window-resize handler skips it for the same
+ * reason. A solo window is exempt: it IS the owner. `_maybeRefetchFullHistory`
+ * already stood aside on the same condition, so this follows a rule the code
+ * had already established.
+ *
+ * Loaded via `vm` with a stubbed context (no jsdom — jsdom is broken on this
+ * box; see connection-indicator.test.ts), the same way terminal-buffer-flush
+ * extracts the real mixin methods from terminal-ui.js.
+ */
+import { readFileSync } from 'node:fs';
+import { performance } from 'node:perf_hooks';
+import { resolve } from 'node:path';
+import vm from 'node:vm';
+import { describe, expect, it, vi } from 'vitest';
+
+/** The mixin runs inside the vm context, so its `fetch` must live there too. */
+let currentFetch: ReturnType<typeof vi.fn> = vi.fn();
+
+function loadTerminalMixin(): Record<string, unknown> {
+  const source = readFileSync(resolve(import.meta.dirname, '../src/web/public/terminal-ui.js'), 'utf8');
+  const FakeCodemanApp = function () {} as unknown as { prototype: Record<string, unknown> };
+  const context = vm.createContext({
+    console,
+    performance,
+    setTimeout,
+    clearTimeout,
+    setInterval: vi.fn(),
+    clearInterval: vi.fn(),
+    requestAnimationFrame: vi.fn(),
+    CodemanApp: FakeCodemanApp,
+    window: { addEventListener: vi.fn(), removeEventListener: vi.fn(), innerWidth: 1600 },
+    document: { addEventListener: vi.fn() },
+    fetch: (...args: unknown[]) => currentFetch(...args),
+  });
+  vm.runInContext(source, context);
+  return FakeCodemanApp.prototype;
+}
+
+const mixin = loadTerminalMixin();
+
+const SESSION = 'session-A';
+
+function makeApp(overrides: Record<string, unknown> = {}) {
+  const fetchMock = vi.fn(async () => ({ json: async () => ({ data: { changed: true } }) }));
+  currentFetch = fetchMock;
+  const app = {
+    sendResize: mixin.sendResize,
+    getTerminalDimensions: () => ({ cols: 120, rows: 40 }),
+    fitAddon: { fit: vi.fn() },
+    detachedSessions: new Set<string>(),
+    isSoloWindow: false,
+    _lastResizeDims: null as { cols: number; rows: number } | null,
+    _wsReady: false,
+    _wsSessionId: null as string | null,
+    ...overrides,
+  } as Record<string, unknown> & { sendResize: (id: string, o?: object) => Promise<boolean> };
+  return { app, fetchMock };
+}
+
+describe('detached sessions own their pane size', () => {
+  it('the dashboard does not resize a session showing in its own window', async () => {
+    const { app, fetchMock } = makeApp();
+    (app.detachedSessions as Set<string>).add(SESSION);
+    const changed = await app.sendResize(SESSION);
+
+    expect(changed).toBe(false);
+    // No measurement and no request: the popup's size stands.
+    expect(fetchMock).not.toHaveBeenCalled();
+    expect((app.fitAddon as { fit: ReturnType<typeof vi.fn> }).fit).not.toHaveBeenCalled();
+  });
+
+  it('the solo window still sizes the session it displays', async () => {
+    const { app, fetchMock } = makeApp({ isSoloWindow: true });
+    (app.detachedSessions as Set<string>).add(SESSION);
+    await app.sendResize(SESSION);
+
+    // The popup is the owner, so being marked detached must not stop it.
+    expect(fetchMock).toHaveBeenCalledTimes(1);
+    expect((app.fitAddon as { fit: ReturnType<typeof vi.fn> }).fit).toHaveBeenCalled();
+  });
+
+  it('the dashboard resizes a session that is not detached', async () => {
+    const { app, fetchMock } = makeApp();
+    await app.sendResize(SESSION);
+
+    expect(fetchMock).toHaveBeenCalledTimes(1);
+  });
+});