diff --git a/apps/web/components/task/dockview-session-tabs.ts b/apps/web/components/task/dockview-session-tabs.ts
index 1b26cfbaaa..ed1628a189 100644
--- a/apps/web/components/task/dockview-session-tabs.ts
+++ b/apps/web/components/task/dockview-session-tabs.ts
@@ -3,6 +3,7 @@ import type { DockviewReadyEvent, AddPanelOptions } from "dockview-react";
 import type { StoreApi } from "zustand";
 import type { AppState } from "@/lib/state/store";
 import { useDockviewStore } from "@/lib/state/dockview-store";
+import { focusOrAddPanel } from "@/lib/state/dockview-layout-builders";
 import { useAppStore } from "@/components/state-provider";
 import { wasPRPanelOffered, markPRPanelOffered } from "@/lib/local-storage";
 
@@ -138,12 +139,14 @@ export function useAutoPRPanel() {
 
         if (decision === "add") {
           const { centerGroupId } = useDockviewStore.getState();
-          const centerGroupExists = centerGroupId && api.groups.some((g) => g.id === centerGroupId);
-          api.addPanel({
+          // Route through focusOrAddPanel so a stale centerGroupId falls back
+          // to another non-sidebar group rather than letting dockview place
+          // the panel in the active group (which may be the sidebar).
+          focusOrAddPanel(api, {
             id: "pr-detail",
             component: "pr-detail",
             title: "Pull Request",
-            position: centerGroupExists ? { referenceGroup: centerGroupId } : undefined,
+            position: { referenceGroup: centerGroupId },
             inactive: true,
           });
           markPRPanelOffered(sessionId);
diff --git a/apps/web/lib/state/dockview-layout-builders.test.ts b/apps/web/lib/state/dockview-layout-builders.test.ts
new file mode 100644
index 0000000000..9dd87b09eb
--- /dev/null
+++ b/apps/web/lib/state/dockview-layout-builders.test.ts
@@ -0,0 +1,73 @@
+import { describe, it, expect } from "vitest";
+import type { DockviewApi } from "dockview-react";
+import { fallbackGroupPosition } from "./dockview-layout-builders";
+import { SIDEBAR_GROUP, CENTER_GROUP, RIGHT_TOP_GROUP, RIGHT_BOTTOM_GROUP } from "./layout-manager";
+
+type MockGroup = { id: string };
+type MockPanel = { id: string; group: { id: string } };
+
+function makeApi(groupIds: string[], panels: MockPanel[] = []): DockviewApi {
+  const groups: MockGroup[] = groupIds.map((id) => ({ id }));
+  return {
+    groups,
+    panels,
+    getPanel: (id: string) => panels.find((p) => p.id === id) ?? undefined,
+  } as unknown as DockviewApi;
+}
+
+describe("fallbackGroupPosition", () => {
+  it("returns the center group when it exists", () => {
+    const api = makeApi([SIDEBAR_GROUP, CENTER_GROUP, "group-other"]);
+
+    expect(fallbackGroupPosition(api)).toEqual({ referenceGroup: CENTER_GROUP });
+  });
+
+  it("returns the chat panel's group even when right groups iterate first", () => {
+    // Drag-to-split can replace the well-known center group ID with a generated one.
+    // Right groups appear before the chat group in iteration order; the fallback
+    // must still prefer the chat group over right-column groups.
+    const chatGroupId = "group-3";
+    const api = makeApi(
+      [SIDEBAR_GROUP, RIGHT_TOP_GROUP, RIGHT_BOTTOM_GROUP, chatGroupId],
+      [{ id: "chat", group: { id: chatGroupId } }],
+    );
+
+    expect(fallbackGroupPosition(api)).toEqual({ referenceGroup: chatGroupId });
+  });
+
+  it("returns the session:* panel's group when no chat panel exists", () => {
+    // Active session: chat is replaced with session:<id>. CENTER_GROUP id was lost.
+    // Right groups iterate first; fallback must still prefer the session group.
+    const sessionGroupId = "group-7";
+    const api = makeApi(
+      [SIDEBAR_GROUP, RIGHT_TOP_GROUP, sessionGroupId],
+      [{ id: "session:abc", group: { id: sessionGroupId } }],
+    );
+
+    expect(fallbackGroupPosition(api)).toEqual({ referenceGroup: sessionGroupId });
+  });
+
+  it("does not return a right-column group when no center-like group exists", () => {
+    // Right-column groups (Changes/Files/Terminal) are tool columns — placing
+    // a diff or PR panel there is the same UX bug as placing it in the sidebar.
+    // With only sidebar+right groups present and no chat/session panels, the
+    // fallback must drop the position so dockview doesn't pick a right group.
+    const api = makeApi([SIDEBAR_GROUP, RIGHT_TOP_GROUP, RIGHT_BOTTOM_GROUP]);
+
+    expect(fallbackGroupPosition(api)).toBeUndefined();
+  });
+
+  it("returns undefined when only the sidebar group exists", () => {
+    // Must NOT return the sidebar group — panels added to the locked sidebar
+    // would leak there, which is the bug we're fixing.
+    const api = makeApi([SIDEBAR_GROUP]);
+
+    expect(fallbackGroupPosition(api)).toBeUndefined();
+  });
+
+  it("returns undefined when no groups exist", () => {
+    const api = makeApi([]);
+
+    expect(fallbackGroupPosition(api)).toBeUndefined();
+  });
+});
diff --git a/apps/web/lib/state/dockview-layout-builders.ts b/apps/web/lib/state/dockview-layout-builders.ts
index a5ca6552d4..4b70217e7d 100644
--- a/apps/web/lib/state/dockview-layout-builders.ts
+++ b/apps/web/lib/state/dockview-layout-builders.ts
@@ -43,17 +43,34 @@ export function applyLayoutFixups(api: DockviewApi): LayoutGroupIds {
 
 /**
  * Resolve a fallback group position when the intended reference is stale.
- * Tries the well-known center group, then any non-sidebar group, then the
- * first group. Returns undefined if no groups exist (drops the position).
+ *
+ * Tries to land in the center column, in this order:
+ *   1. Well-known CENTER_GROUP id.
+ *   2. Group containing the `chat` panel (post-drag, the well-known id may be
+ *      gone but the chat panel still marks the center column).
+ *   3. Group containing any `session:*` panel (active session: chat is removed
+ *      and replaced with per-session tabs).
+ *   4. Any group that is NOT the sidebar AND NOT a right-column group
+ *      (Changes/Files/Terminal). Returning a right-column group would leak the
+ *      panel into the narrow tools column — same UX bug as the sidebar leak.
+ *
+ * Returns undefined if no center-like group exists. The caller drops the
+ * position so dockview picks a default. Never returns the sidebar.
  */
-function fallbackGroupPosition(api: DockviewApi): { referenceGroup: string } | undefined {
+export function fallbackGroupPosition(api: DockviewApi): { referenceGroup: string } | undefined {
   const centerGroup = api.groups.find((g) => g.id === CENTER_GROUP);
   if (centerGroup) return { referenceGroup: centerGroup.id };
 
-  const nonSidebarGroup = api.groups.find((g) => g.id !== SIDEBAR_GROUP);
-  if (nonSidebarGroup) return { referenceGroup: nonSidebarGroup.id };
+  const chatGroupId = api.getPanel("chat")?.group?.id;
+  if (chatGroupId) return { referenceGroup: chatGroupId };
 
-  if (api.groups.length > 0) return { referenceGroup: api.groups[0].id };
+  const sessionGroupId = api.panels.find((p) => p.id.startsWith("session:"))?.group?.id;
+  if (sessionGroupId) return { referenceGroup: sessionGroupId };
+
+  const centerish = api.groups.find(
+    (g) => g.id !== SIDEBAR_GROUP && g.id !== RIGHT_TOP_GROUP && g.id !== RIGHT_BOTTOM_GROUP,
+  );
+  if (centerish) return { referenceGroup: centerish.id };
 
   return undefined;
 }
diff --git a/apps/web/lib/state/layout-manager/applier.test.ts b/apps/web/lib/state/layout-manager/applier.test.ts
new file mode 100644
index 0000000000..b8c5f88bda
--- /dev/null
+++ b/apps/web/lib/state/layout-manager/applier.test.ts
@@ -0,0 +1,71 @@
+import { describe, it, expect } from "vitest";
+import type { DockviewApi } from "dockview-react";
+import { resolveGroupIds } from "./applier";
+import { SIDEBAR_GROUP, CENTER_GROUP, RIGHT_TOP_GROUP, RIGHT_BOTTOM_GROUP } from "./constants";
+
+type MockGroup = { id: string };
+type MockPanel = { id: string; group: { id: string } };
+
+function makeApi(groups: MockGroup[], panels: MockPanel[] = []): DockviewApi {
+  return {
+    groups,
+    panels,
+    getPanel: (id: string) => panels.find((p) => p.id === id) ?? undefined,
+  } as unknown as DockviewApi;
+}
+
+describe("resolveGroupIds", () => {
+  it("returns well-known IDs when all groups exist", () => {
+    const api = makeApi([
+      { id: SIDEBAR_GROUP },
+      { id: CENTER_GROUP },
+      { id: RIGHT_TOP_GROUP },
+      { id: RIGHT_BOTTOM_GROUP },
+    ]);
+
+    const ids = resolveGroupIds(api);
+
+    expect(ids.sidebarGroupId).toBe(SIDEBAR_GROUP);
+    expect(ids.centerGroupId).toBe(CENTER_GROUP);
+    expect(ids.rightTopGroupId).toBe(RIGHT_TOP_GROUP);
+    expect(ids.rightBottomGroupId).toBe(RIGHT_BOTTOM_GROUP);
+  });
+
+  it("falls back to chat panel's group when CENTER_GROUP id missing", () => {
+    // Simulates post-drag state where center group has a dockview-generated ID
+    const chatGroupId = "group-5";
+    const api = makeApi(
+      [{ id: SIDEBAR_GROUP }, { id: chatGroupId }],
+      [{ id: "chat", group: { id: chatGroupId } }],
+    );
+
+    const ids = resolveGroupIds(api);
+
+    expect(ids.centerGroupId).toBe(chatGroupId);
+  });
+
+  it("falls back to session:* panel's group when no chat panel exists", () => {
+    // Active session: chat panel was removed, replaced with session:<id>
+    // CENTER_GROUP id was lost (e.g. drag-to-split). This is the bug scenario.
+    const sessionGroupId = "group-7";
+    const api = makeApi(
+      [{ id: SIDEBAR_GROUP }, { id: sessionGroupId }],
+      [{ id: "session:abc123", group: { id: sessionGroupId } }],
+    );
+
+    const ids = resolveGroupIds(api);
+
+    expect(ids.centerGroupId).toBe(sessionGroupId);
+  });
+
+  it("returns the CENTER_GROUP constant as last-resort when nothing matches", () => {
+    // Last-resort fallback: returns the well-known constant even when no live
+    // group carries that ID. The caller (focusOrAddPanel) detects the stale ID
+    // and applies its own fallback via fallbackGroupPosition.
+    const api = makeApi([{ id: SIDEBAR_GROUP }], []);
+
+    const ids = resolveGroupIds(api);
+
+    expect(ids.centerGroupId).toBe(CENTER_GROUP);
+  });
+});
diff --git a/apps/web/lib/state/layout-manager/applier.ts b/apps/web/lib/state/layout-manager/applier.ts
index b644b82045..9023403bfa 100644
--- a/apps/web/lib/state/layout-manager/applier.ts
+++ b/apps/web/lib/state/layout-manager/applier.ts
@@ -32,10 +32,23 @@ function findGroupId(api: DockviewApi, knownId: string, fallbackPanelId: string)
   return pnl?.group?.id ?? knownId;
 }
 
+/** Find the center group, preferring the well-known ID, then "chat", then any
+ *  "session:*" panel's group. When a session is active, "chat" is removed and
+ *  replaced with per-session tabs — without the session fallback the returned
+ *  ID would be a stale constant that doesn't match any live group. */
+function findCenterGroupId(api: DockviewApi): string {
+  if (api.groups.some((g) => g.id === CENTER_GROUP)) return CENTER_GROUP;
+  const chat = api.getPanel("chat");
+  if (chat?.group?.id) return chat.group.id;
+  const sessionPanel = api.panels.find((p) => p.id.startsWith("session:"));
+  if (sessionPanel?.group?.id) return sessionPanel.group.id;
+  return CENTER_GROUP;
+}
+
 export function resolveGroupIds(api: DockviewApi): LayoutGroupIds {
   return {
     sidebarGroupId: findGroupId(api, SIDEBAR_GROUP, "sidebar"),
-    centerGroupId: findGroupId(api, CENTER_GROUP, "chat"),
+    centerGroupId: findCenterGroupId(api),
     // Always use the well-known constant — do NOT fall back to the "changes"
     // panel's current group. In plan mode the "changes" panel moves into the
     // center group; a panel-based fallback would return the center group ID and