diff --git a/packages/studio/src/hooks/usePanelLayout.test.ts b/packages/studio/src/hooks/usePanelLayout.test.ts
index 2106b02b6..b1ddf2fa1 100644
--- a/packages/studio/src/hooks/usePanelLayout.test.ts
+++ b/packages/studio/src/hooks/usePanelLayout.test.ts
@@ -2,23 +2,25 @@
 
 import React, { act } from "react";
 import { createRoot } from "react-dom/client";
-import { afterEach, describe, expect, it } from "vitest";
+import { afterEach, describe, expect, it, vi } from "vitest";
 import { usePanelLayout } from "./usePanelLayout";
 
 (globalThis as unknown as { IS_REACT_ACT_ENVIRONMENT: boolean }).IS_REACT_ACT_ENVIRONMENT = true;
 
 afterEach(() => {
   document.body.innerHTML = "";
+  vi.doUnmock("../components/editor/manualEditingAvailability");
+  vi.resetModules();
 });
 
-function renderPanelLayout() {
+function renderPanelLayoutWith(hook: typeof usePanelLayout) {
   const host = document.createElement("div");
   document.body.append(host);
   const root = createRoot(host);
   let current: ReturnType<typeof usePanelLayout> | null = null;
 
   function Harness() {
-    current = usePanelLayout();
+    current = hook();
     return null;
   }
 
@@ -35,6 +37,10 @@ function renderPanelLayout() {
   };
 }
 
+function renderPanelLayout() {
+  return renderPanelLayoutWith(usePanelLayout);
+}
+
 describe("usePanelLayout — right inspector panes", () => {
   it("toggleRightInspectorPane independently flips one pane, allowing both open at once", () => {
     const harness = renderPanelLayout();
@@ -67,4 +73,46 @@ describe("usePanelLayout — right inspector panes", () => {
 
     harness.unmount();
   });
+
+  it("setRightPanelTab additively opens a pane when the flat inspector is off (legacy behavior)", async () => {
+    vi.resetModules();
+    vi.doMock("../components/editor/manualEditingAvailability", async () => {
+      const actual = await vi.importActual<
+        typeof import("../components/editor/manualEditingAvailability")
+      >("../components/editor/manualEditingAvailability");
+      return { ...actual, STUDIO_FLAT_INSPECTOR_ENABLED: false };
+    });
+    const { usePanelLayout: usePanelLayoutFlatOff } = await import("./usePanelLayout");
+    const harness = renderPanelLayoutWith(usePanelLayoutFlatOff);
+    expect(harness.getState().rightInspectorPanes).toEqual({ layers: false, design: true });
+
+    act(() => harness.getState().setRightPanelTab("layers"));
+    // Legacy (split-view) behavior: additive, both panes end up open.
+    expect(harness.getState().rightInspectorPanes).toEqual({ layers: true, design: true });
+
+    harness.unmount();
+  });
+
+  it("setRightPanelTab is flat-aware: exclusivity holds for callers other than a direct in-panel tab click", async () => {
+    vi.resetModules();
+    vi.doMock("../components/editor/manualEditingAvailability", async () => {
+      const actual = await vi.importActual<
+        typeof import("../components/editor/manualEditingAvailability")
+      >("../components/editor/manualEditingAvailability");
+      return { ...actual, STUDIO_FLAT_INSPECTOR_ENABLED: true };
+    });
+    const { usePanelLayout: usePanelLayoutFlatOn } = await import("./usePanelLayout");
+    const harness = renderPanelLayoutWith(usePanelLayoutFlatOn);
+    expect(harness.getState().rightInspectorPanes).toEqual({ layers: false, design: true });
+
+    // Element-select / block-params-close / header Inspector-button callers
+    // all reach setRightPanelTab directly, not through the in-panel tab
+    // click's own setExclusiveRightInspectorPane call — this must still
+    // enforce exclusivity under the flat flag, or both tabs end up
+    // highlighted while only one pane actually renders.
+    act(() => harness.getState().setRightPanelTab("layers"));
+    expect(harness.getState().rightInspectorPanes).toEqual({ layers: true, design: false });
+
+    harness.unmount();
+  });
 });
diff --git a/packages/studio/src/hooks/usePanelLayout.ts b/packages/studio/src/hooks/usePanelLayout.ts
index 2e0420e72..df94b244e 100644
--- a/packages/studio/src/hooks/usePanelLayout.ts
+++ b/packages/studio/src/hooks/usePanelLayout.ts
@@ -6,6 +6,7 @@ import type {
 } from "../utils/studioHelpers";
 import { readStudioUiPreferences, writeStudioUiPreferences } from "../utils/studioUiPreferences";
 import { trackStudioEvent } from "../utils/studioTelemetry";
+import { STUDIO_FLAT_INSPECTOR_ENABLED } from "../components/editor/manualEditingAvailability";
 
 export interface InitialPanelLayoutState {
   rightCollapsed?: boolean | null;
@@ -80,7 +81,20 @@ export function usePanelLayout(initialState?: InitialPanelLayoutState) {
   const trackedSetRightPanelTab = useCallback(
     (tab: RightPanelTab) => {
       if (tab === "design" || tab === "layers") {
-        setRightInspectorPanes((panes) => ({ ...panes, [tab]: true }));
+        // Flat inspector: Layers always renders full-height by itself (see
+        // StudioRightPanel's render gate), so this MUST land on the same
+        // radio-style exclusivity setExclusiveRightInspectorPane enforces for
+        // the direct in-panel tab click — every OTHER path that reaches here
+        // (element select, closing block-params, the header Inspector
+        // button, and this function's own callers outside an active
+        // inspector tab) would otherwise additively leave both panes `true`
+        // and reproduce the "both tabs highlight, only one renders" bug this
+        // still-additive branch used to cause under the flat flag.
+        setRightInspectorPanes(
+          STUDIO_FLAT_INSPECTOR_ENABLED
+            ? { design: tab === "design", layers: tab === "layers" }
+            : (panes) => ({ ...panes, [tab]: true }),
+        );
       }
       setRightPanelTab(tab);
       trackStudioEvent("tab_switch", { panel: "right_panel", tab });