feat(tabs): confirm dialog that cascades a tab close to its subagents

Closing a tab whose visible subagent group is non-empty now asks ONE
daisyUI modal (CloseSubagentsConfirm): 'Yes, close all N' closes the tab
plus every transitive descendant tab (sub-sub-agents included, no further
prompts), 'No, keep them open' closes only the tab, and Cancel/Escape/
backdrop closes nothing. Plain tabs close directly as before.

- cascadeCloseIds (pure, tab-groups.ts): the tab + its transitive visible
  group descendants in tab order — one decision covers the whole subtree
- TabList intercepts tab-close and opens the dialog only when the
  subtree has >1 tab; ids are snapshotted at open time
- each closed conversation goes through the existing onClose port, so
  every close still POSTs /conversations/:id/close (backend doesn't
  cascade; it is idempotent)

6 files changed, 218 insertions, 2 deletions

diff --git a/src/features/tabs/index.ts b/src/features/tabs/index.ts
index 980e8ee..0094528 100644
--- a/src/features/tabs/index.ts
+++ b/src/features/tabs/index.ts
@@ -1,6 +1,7 @@
 export type { TabGroupForest, TabGroupNode } from "./tab-groups";
 export {
   buildTabGroups,
+  cascadeCloseIds,
   directGroupChildren,
   groupBleedPx,
   groupDescendants,
diff --git a/src/features/tabs/tab-groups.test.ts b/src/features/tabs/tab-groups.test.ts
index 83f273c..a89a469 100644
--- a/src/features/tabs/tab-groups.test.ts
+++ b/src/features/tabs/tab-groups.test.ts
@@ -1,6 +1,7 @@
 import { describe, expect, it } from "vitest";
 import {
   buildTabGroups,
+  cascadeCloseIds,
   directGroupChildren,
   groupBleedPx,
   groupDescendants,
@@ -103,6 +104,34 @@ describe("groupDescendants / directGroupChildren / groupParentCandidates", () =>
   });
 });
 
+describe("cascadeCloseIds (the ids a cascading tab close removes)", () => {
+  const tabs = [
+    tab("p"),
+    tab("c1", "p"),
+    tab("c2", "p"),
+    tab("g", "c1"), // sub-sub-agent: grouped under c1, still cascades from p
+    tab("solo"),
+  ];
+
+  it("returns the tab first, then its transitive descendants in tab order", () => {
+    expect(cascadeCloseIds(tabs, "p")).toEqual(["p", "c1", "c2", "g"]);
+  });
+
+  it("starts the cascade at a mid-level tab, taking only its own subtree", () => {
+    expect(cascadeCloseIds(tabs, "c1")).toEqual(["c1", "g"]);
+  });
+
+  it("returns just the tab itself when it has no subagents (closes without a prompt)", () => {
+    expect(cascadeCloseIds(tabs, "solo")).toEqual(["solo"]);
+    expect(cascadeCloseIds(tabs, "c2")).toEqual(["c2"]);
+  });
+
+  it("ignores tabs whose group parent is no longer open", () => {
+    const stale = [tab("p"), tab("orphan", "gone")];
+    expect(cascadeCloseIds(stale, "p")).toEqual(["p"]);
+  });
+});
+
 describe("groupBleedPx / groupGutterPx", () => {
   it("is zero without groups", () => {
     expect(groupGutterPx(0)).toBe(0);
diff --git a/src/features/tabs/tab-groups.ts b/src/features/tabs/tab-groups.ts
index 12870ee..a1aa464 100644
--- a/src/features/tabs/tab-groups.ts
+++ b/src/features/tabs/tab-groups.ts
@@ -104,6 +104,22 @@ export function groupDescendants(tabs: readonly Tab[], conversationId: string):
 }
 
 /**
+ * The tabs removed by a CASCADING close of `conversationId` (the user chose
+ * "also close its subagents"): the tab itself first, then its transitive
+ * visible-group descendants in tab order. ONE decision covers the whole
+ * subtree — closing never re-prompts per level. A single-element result (the
+ * tab alone) means the tab has no subagents and closes without a prompt.
+ */
+export function cascadeCloseIds(tabs: readonly Tab[], conversationId: string): string[] {
+  const descendants = groupDescendants(tabs, conversationId);
+  const ids = [conversationId];
+  for (const tab of tabs) {
+    if (descendants.has(tab.conversationId)) ids.push(tab.conversationId);
+  }
+  return ids;
+}
+
+/**
  * The tabs `conversationId` may be moved under (the "Add to group" menu):
  * every other tab EXCEPT itself, its descendants (that would form a cycle),
  * and its current visible group parent (already there). Tab order preserved.
diff --git a/src/features/tabs/ui.test.ts b/src/features/tabs/ui.test.ts
index 2569438..0243ea4 100644
--- a/src/features/tabs/ui.test.ts
+++ b/src/features/tabs/ui.test.ts
@@ -1,4 +1,4 @@
-import { fireEvent, render, screen } from "@testing-library/svelte";
+import { fireEvent, render, screen, within } from "@testing-library/svelte";
 import userEvent from "@testing-library/user-event";
 import { describe, expect, it, vi } from "vitest";
 import type { Tab } from "./tabs";
@@ -544,5 +544,77 @@ describe("TabList", () => {
         vi.useRealTimers();
       }
     });
+
+    describe("closing a tab with subagents (confirm + cascade)", () => {
+      const dialog = (): HTMLElement =>
+        screen.getByRole("dialog", { name: "Close subagents too?" });
+
+      async function clickCloseOn(id: string): Promise<void> {
+        await fireEvent.click(within(card(id)).getByRole("button", { name: "Close tab" }));
+      }
+
+      function renderClosable(onClose = vi.fn()) {
+        render(TabList, {
+          props: {
+            tabs: grouped,
+            activeConversationId: "p",
+            onSelect: vi.fn(),
+            onClose,
+            onNewDraft: vi.fn(),
+          },
+        });
+        return onClose;
+      }
+
+      it("closes a tab WITHOUT subagents directly — no confirm dialog", async () => {
+        const onClose = renderClosable();
+        await clickCloseOn("solo");
+        expect(onClose).toHaveBeenCalledTimes(1);
+        expect(onClose).toHaveBeenCalledWith("solo");
+        expect(screen.queryByRole("dialog")).toBeNull();
+      });
+
+      it("shows ONE confirm dialog for a group parent (never one per descendant)", async () => {
+        const onClose = renderClosable();
+        await clickCloseOn("p");
+        // 3 subagent tabs: c1, c2 and the nested grandchild g.
+        expect(dialog()).toHaveTextContent("3 subagent tabs");
+        expect(onClose).not.toHaveBeenCalled();
+      });
+
+      it("'No, keep them open' closes only the tab itself", async () => {
+        const onClose = renderClosable();
+        await clickCloseOn("p");
+        await fireEvent.click(screen.getByRole("button", { name: /No, keep them open/ }));
+        expect(onClose).toHaveBeenCalledTimes(1);
+        expect(onClose).toHaveBeenCalledWith("p");
+        expect(screen.queryByRole("dialog")).toBeNull();
+      });
+
+      it("'Yes, close all' cascades to every descendant — no further prompts", async () => {
+        const onClose = renderClosable();
+        await clickCloseOn("p");
+        await fireEvent.click(screen.getByRole("button", { name: /Yes, close all/ }));
+        // p first, then its transitive descendants in tab order (c1, c2, g).
+        expect(onClose.mock.calls.map((c) => c[0])).toEqual(["p", "c1", "c2", "g"]);
+        expect(screen.queryByRole("dialog")).toBeNull();
+      });
+
+      it("cascades from a mid-level subagent, taking only its own subtree", async () => {
+        const onClose = renderClosable();
+        await clickCloseOn("c1");
+        expect(dialog()).toHaveTextContent("1 subagent tab");
+        await fireEvent.click(screen.getByRole("button", { name: /Yes, close all/ }));
+        expect(onClose.mock.calls.map((c) => c[0])).toEqual(["c1", "g"]);
+      });
+
+      it("Escape dismisses the dialog without closing anything", async () => {
+        const onClose = renderClosable();
+        await clickCloseOn("p");
+        await fireEvent.keyDown(window, { key: "Escape" });
+        expect(screen.queryByRole("dialog")).toBeNull();
+        expect(onClose).not.toHaveBeenCalled();
+      });
+    });
   });
 });
diff --git a/src/features/tabs/ui/CloseSubagentsConfirm.svelte b/src/features/tabs/ui/CloseSubagentsConfirm.svelte
new file mode 100644
index 0000000..cbbec79
--- /dev/null
+++ b/src/features/tabs/ui/CloseSubagentsConfirm.svelte
@@ -0,0 +1,57 @@
+<script lang="ts">
+  import type { Tab } from "../tabs";
+
+  /**
+   * The close-subagents confirm dialog: shown when the user closes a tab that
+   * still has subagent tabs in its (visible) subagent group. ONE dialog for
+   * the whole subtree — "Yes" cascades the close to every transitive
+   * descendant (sub-sub-agents included, no further prompts); "No" closes
+   * only the tab itself. Escape / backdrop dismisses without closing anything.
+   * daisyUI `modal` (controlled: `modal-open`).
+   */
+  let {
+    tab,
+    subagentCount,
+    onKeep,
+    onCloseAll,
+    onDismiss,
+  }: {
+    /** The tab being closed (drives the title shown in the dialog). */
+    tab: Tab;
+    /** Number of tabs in the tab's subagent group (transitive). */
+    subagentCount: number;
+    /** "No" — close only this tab, keep the subagents open. */
+    onKeep: () => void;
+    /** "Yes" — close this tab AND cascade to all its subagents. */
+    onCloseAll: () => void;
+    /** Escape / backdrop — close nothing. */
+    onDismiss: () => void;
+  } = $props();
+</script>
+
+<svelte:window onkeydown={(e) => e.key === "Escape" && onDismiss()} />
+
+<div class="modal modal-open" role="dialog" aria-label="Close subagents too?">
+  <div class="modal-box max-w-md">
+    <h3 class="text-lg font-bold">Close subagents too?</h3>
+    <div class="flex flex-col gap-1 py-3">
+      <p class="text-sm">
+        "{tab.title}" has {subagentCount}
+        {subagentCount === 1 ? "subagent tab" : "subagent tabs"} (including nested ones).
+      </p>
+      <p class="text-sm">
+        Close them as well? Their own subagents are closed too, and any in-flight turns are
+        stopped.
+      </p>
+    </div>
+    <div class="modal-action">
+      <button type="button" class="btn btn-ghost btn-sm" onclick={onDismiss}>Cancel</button>
+      <button type="button" class="btn btn-sm" onclick={onKeep}>No, keep them open</button>
+      <button type="button" class="btn btn-warning btn-sm" onclick={onCloseAll}>
+        Yes, close all {subagentCount + 1}
+      </button>
+    </div>
+  </div>
+  <!-- svelte-ignore a11y_click_events_have_key_events, a11y_no_static_element_interactions -->
+  <div class="modal-backdrop" onclick={onDismiss}></div>
+</div>
diff --git a/src/features/tabs/ui/TabList.svelte b/src/features/tabs/ui/TabList.svelte
index 06d5aa6..4b96e08 100644
--- a/src/features/tabs/ui/TabList.svelte
+++ b/src/features/tabs/ui/TabList.svelte
@@ -1,6 +1,7 @@
 <script lang="ts">
   import {
     buildTabGroups,
+    cascadeCloseIds,
     directGroupChildren,
     groupGutterPx,
     groupParentCandidates,
@@ -8,6 +9,7 @@
   } from "../tab-groups";
   import type { Tab } from "../tabs";
   import { shortHandle } from "../tabs";
+  import CloseSubagentsConfirm from "./CloseSubagentsConfirm.svelte";
   import TabCard from "./TabCard.svelte";
   import TabGroupBox from "./TabGroupBox.svelte";
   import TabMenu from "./TabMenu.svelte";
@@ -72,6 +74,26 @@
     if (onSetTabGroup === undefined) return;
     menu = { conversationId, x, y };
   }
+
+  // Close-with-confirm: a tab whose (visible) subagent group is non-empty
+  // gets ONE confirm dialog covering its whole subtree (never one per level).
+  // The dialog's ids are snapshotted at open time; a tab the backend closed
+  // in the meantime is a no-op for `onClose`. A plain tab closes directly.
+  let closeConfirm = $state<{ tab: Tab; cascadeIds: readonly string[] } | null>(null);
+
+  function requestClose(conversationId: string): void {
+    const cascadeIds = cascadeCloseIds(tabs, conversationId);
+    if (cascadeIds.length <= 1) {
+      onClose(conversationId);
+      return;
+    }
+    const tab = tabs.find((t) => t.conversationId === conversationId);
+    if (tab === undefined) {
+      onClose(conversationId);
+      return;
+    }
+    closeConfirm = { tab, cascadeIds };
+  }
 </script>
 
 {#snippet card(tab: Tab, wide: boolean, subagentCount: number)}
@@ -83,7 +105,7 @@
     {wide}
     {subagentCount}
     {onSelect}
-    {onClose}
+    onClose={requestClose}
     {onRename}
     onMenu={openMenu}
   />
@@ -147,3 +169,22 @@
     onDismiss={() => (menu = null)}
   />
 {/if}
+
+{#if closeConfirm !== null}
+  {@const target = closeConfirm.tab}
+  {@const cascadeIds = [...closeConfirm.cascadeIds]}
+  {@const closeOwner = onClose}
+  <CloseSubagentsConfirm
+    tab={target}
+    subagentCount={cascadeIds.length - 1}
+    onKeep={() => {
+      closeOwner(target.conversationId);
+      closeConfirm = null;
+    }}
+    onCloseAll={() => {
+      for (const id of cascadeIds) closeOwner(id);
+      closeConfirm = null;
+    }}
+    onDismiss={() => (closeConfirm = null)}
+  />
+{/if}