diff --git a/apps/app/src/components/secondary-panel/SecondaryPanelLayout.split-drag.test.tsx b/apps/app/src/components/secondary-panel/SecondaryPanelLayout.split-drag.test.tsx
new file mode 100644
index 0000000000..8f47504b6a
--- /dev/null
+++ b/apps/app/src/components/secondary-panel/SecondaryPanelLayout.split-drag.test.tsx
@@ -0,0 +1,110 @@
+// @vitest-environment jsdom
+
+import { cleanup, fireEvent, render, screen } from "@testing-library/react";
+import { afterEach, expect, it, vi } from "vitest";
+import { CompactViewportOverrideProvider } from "@bb/shared-ui/hooks/use-compact-viewport";
+import {
+  PaneContext,
+  type PaneContextValue,
+} from "@/views/thread-detail/PaneContext";
+import { beginSplitDrag, decideThreadDrop } from "@/lib/split-drag";
+import { SecondaryPanelLayout } from "./SecondaryPanelLayout";
+
+const noop = () => {};
+const context: PaneContextValue = {
+  paneId: "pane-test",
+  isFocused: true,
+  isSplitPane: false,
+  secondaryPanelHost: null,
+  reservesWindowPanelToggle: false,
+  onRequestClose: null,
+  isMaximized: false,
+  onToggleMaximize: null,
+  isBoundedPane: false,
+  isTopRow: true,
+  ownsWindowTopLeft: true,
+  navigateInPane: noop,
+};
+
+afterEach(() => {
+  fireEvent(window, new Event("pointercancel"));
+  cleanup();
+  vi.restoreAllMocks();
+  vi.unstubAllGlobals();
+});
+
+it.each([false, true])(
+  "resolves a single chat's center with panel open=%s",
+  (open) => {
+    render(
+      <main data-testid="workspace">
+        <PaneContext.Provider value={context}>
+          <CompactViewportOverrideProvider isCompactViewport={false}>
+            <SecondaryPanelLayout
+              open={open}
+              onToggle={noop}
+              onClose={noop}
+              resetKey="test"
+              contentKey="test"
+              drawerLabel="Details"
+              drawerFallback={null}
+              mainPanelId="chat"
+              main={<div data-testid="chat">Chat</div>}
+              renderPanel={() => <div data-testid="details">Details</div>}
+              composerHost={null}
+              compactPresentation="shelf"
+            />
+          </CompactViewportOverrideProvider>
+        </PaneContext.Provider>
+      </main>,
+    );
+    const workspace = screen.getByTestId("workspace");
+    const chat = screen.getByTestId("chat");
+    const width = open ? 500 : 1000;
+    vi.spyOn(HTMLElement.prototype, "getBoundingClientRect").mockImplementation(
+      function (this: HTMLElement) {
+        return new DOMRect(200, 0, this === workspace ? 1000 : width, 600);
+      },
+    );
+    const originalHitTest = Object.getOwnPropertyDescriptor(
+      document,
+      "elementsFromPoint",
+    );
+    Object.defineProperty(document, "elementsFromPoint", {
+      configurable: true,
+      value: () => [chat],
+    });
+    const onDrop = vi.fn();
+    beginSplitDrag({
+      ghostLabel: "Another thread",
+      fallback: { paneId: context.paneId, container: workspace },
+      shouldEngage: () => true,
+      decide: (_paneId, zone) =>
+        decideThreadDrop({ zone, threadAlreadyOpen: false, atMaxPanes: false }),
+      onDrop,
+    });
+    try {
+      fireEvent(
+        window,
+        new MouseEvent("pointermove", {
+          clientX: 200 + width / 2,
+          clientY: 300,
+        }),
+      );
+      expect(
+        document.querySelector("[data-split-drag-label]")?.textContent,
+      ).toBe("Replace this chat");
+      fireEvent(window, new Event("pointerup"));
+      expect(onDrop).toHaveBeenCalledWith({
+        paneId: context.paneId,
+        zone: "center",
+      });
+      const target = chat.closest("[data-split-pane-id]");
+      expect(target?.contains(screen.getByTestId("details"))).toBe(false);
+    } finally {
+      if (originalHitTest)
+        Object.defineProperty(document, "elementsFromPoint", originalHitTest);
+      else Reflect.deleteProperty(document, "elementsFromPoint");
+    }
+  },
+);
diff --git a/apps/app/src/components/secondary-panel/SecondaryPanelLayout.tsx b/apps/app/src/components/secondary-panel/SecondaryPanelLayout.tsx
index bafad99c04..aa603eab46 100644
--- a/apps/app/src/components/secondary-panel/SecondaryPanelLayout.tsx
+++ b/apps/app/src/components/secondary-panel/SecondaryPanelLayout.tsx
@@ -297,6 +297,9 @@ export function SecondaryPanelLayout({
 
   const mainContent = (
     <div
+      data-split-pane-id={
+        secondaryPanelHost === null ? paneContext?.paneId : undefined
+      }
       data-conversation-collapsed={isMainCollapsed}
       inert={isMainCollapsed}
       className={cn(