diff --git a/decnet_web/src/components/MazeNET/MazeNET.tsx b/decnet_web/src/components/MazeNET/MazeNET.tsx
index af2238b7..1cb45d77 100644
--- a/decnet_web/src/components/MazeNET/MazeNET.tsx
+++ b/decnet_web/src/components/MazeNET/MazeNET.tsx
@@ -22,6 +22,7 @@ import { useTopologyEditor } from './useTopologyEditor';
 import { useMazeInteraction, type PaletteDrag } from './useMazeInteraction';
 import { useLayoutPersistor } from './useMazeLayoutStore';
 import { useTopologyStream, type TopologyStreamEvent } from './useTopologyStream';
+import { useFullscreenMode } from './useFullscreenMode';
 import { ARCHETYPES as DEFAULT_ARCHETYPES } from './data';
 import { useToast } from '../Toasts/useToast';
 import { useServiceRegistry } from '../../hooks/useServiceRegistry';
@@ -187,43 +188,7 @@ const MazeNET: React.FC = () => {
   const [selection, setSelection] = useState<Selection>(null);
   const [inspectorOpen, setInspectorOpen] = useState(true);
   const [paletteOpen, setPaletteOpen] = useState(true);
-  const [fullscreen, setFullscreen] = useState(false);
-
-  useEffect(() => {
-    const cls = 'maze-fullscreen';
-    if (fullscreen) document.body.classList.add(cls);
-    else document.body.classList.remove(cls);
-    return () => document.body.classList.remove(cls);
-  }, [fullscreen]);
-
-  // Request/exit browser fullscreen alongside the in-app chrome hide.
-  // Ignore failures (fullscreen requires a user gesture; the chrome-only
-  // mode still works if the API rejects).
-  useEffect(() => {
-    if (fullscreen && !document.fullscreenElement) {
-      document.documentElement.requestFullscreen?.().catch(() => {});
-    } else if (!fullscreen && document.fullscreenElement) {
-      document.exitFullscreen?.().catch(() => {});
-    }
-  }, [fullscreen]);
-
-  // Sync state if the user presses F11/Esc to leave fullscreen from
-  // outside our button.
-  useEffect(() => {
-    const onFsChange = () => {
-      if (!document.fullscreenElement) setFullscreen(false);
-    };
-    document.addEventListener('fullscreenchange', onFsChange);
-    return () => document.removeEventListener('fullscreenchange', onFsChange);
-  }, []);
-
-  useEffect(() => {
-    const onKey = (e: KeyboardEvent) => {
-      if (e.key === 'Escape' && fullscreen) setFullscreen(false);
-    };
-    window.addEventListener('keydown', onKey);
-    return () => window.removeEventListener('keydown', onKey);
-  }, [fullscreen]);
+  const { fullscreen, toggle: toggleFullscreen } = useFullscreenMode();
   const [services, setServices] = useState<ServiceDef[]>(DEFAULT_SERVICES);
   const [archetypes, setArchetypes] = useState<Archetype[]>(DEFAULT_ARCHETYPES);
 
@@ -839,7 +804,7 @@ const MazeNET: React.FC = () => {
           <button
             type="button"
             className="maze-btn ghost"
-            onClick={() => setFullscreen((f) => !f)}
+            onClick={toggleFullscreen}
             title={fullscreen ? 'Exit fullscreen (Esc)' : 'Fullscreen canvas'}
           >
             {fullscreen ? <Minimize2 size={12} /> : <Maximize2 size={12} />}
diff --git a/decnet_web/src/components/MazeNET/useFullscreenMode.test.ts b/decnet_web/src/components/MazeNET/useFullscreenMode.test.ts
new file mode 100644
index 00000000..1494cfc5
--- /dev/null
+++ b/decnet_web/src/components/MazeNET/useFullscreenMode.test.ts
@@ -0,0 +1,48 @@
+/**
+ * @vitest-environment jsdom
+ */
+import { describe, it, expect, beforeEach } from 'vitest';
+import { act, renderHook } from '@testing-library/react';
+import { useFullscreenMode } from './useFullscreenMode';
+
+beforeEach(() => {
+  document.body.classList.remove('maze-fullscreen');
+});
+
+describe('useFullscreenMode', () => {
+  it('starts off and toggles cleanly', () => {
+    const { result } = renderHook(() => useFullscreenMode());
+    expect(result.current.fullscreen).toBe(false);
+    act(() => result.current.toggle());
+    expect(result.current.fullscreen).toBe(true);
+    act(() => result.current.toggle());
+    expect(result.current.fullscreen).toBe(false);
+  });
+
+  it('adds the maze-fullscreen body class only while active', () => {
+    const { result } = renderHook(() => useFullscreenMode());
+    expect(document.body.classList.contains('maze-fullscreen')).toBe(false);
+    act(() => result.current.setFullscreen(true));
+    expect(document.body.classList.contains('maze-fullscreen')).toBe(true);
+    act(() => result.current.setFullscreen(false));
+    expect(document.body.classList.contains('maze-fullscreen')).toBe(false);
+  });
+
+  it('Esc keystroke flips fullscreen back off', () => {
+    const { result } = renderHook(() => useFullscreenMode());
+    act(() => result.current.setFullscreen(true));
+    expect(result.current.fullscreen).toBe(true);
+    act(() => {
+      window.dispatchEvent(new KeyboardEvent('keydown', { key: 'Escape' }));
+    });
+    expect(result.current.fullscreen).toBe(false);
+  });
+
+  it('clears the body class on unmount', () => {
+    const { result, unmount } = renderHook(() => useFullscreenMode());
+    act(() => result.current.setFullscreen(true));
+    expect(document.body.classList.contains('maze-fullscreen')).toBe(true);
+    unmount();
+    expect(document.body.classList.contains('maze-fullscreen')).toBe(false);
+  });
+});
diff --git a/decnet_web/src/components/MazeNET/useFullscreenMode.ts b/decnet_web/src/components/MazeNET/useFullscreenMode.ts
new file mode 100644
index 00000000..0cc79468
--- /dev/null
+++ b/decnet_web/src/components/MazeNET/useFullscreenMode.ts
@@ -0,0 +1,62 @@
+import { useEffect, useState } from 'react';
+
+const BODY_CLASS = 'maze-fullscreen';
+
+export interface UseFullscreenModeResult {
+  fullscreen: boolean;
+  setFullscreen: (next: boolean | ((prev: boolean) => boolean)) => void;
+  toggle: () => void;
+}
+
+/** Fullscreen-mode state for the MazeNET canvas. Owns four
+ *  side-effects:
+ *    1. Toggle a body class so the page CSS can hide its chrome.
+ *    2. Request/exit the browser-level fullscreen API (failures
+ *       are ignored; chrome-only mode still works without it).
+ *    3. Listen for fullscreenchange so F11/Esc from outside our
+ *       button keeps internal state in sync.
+ *    4. Esc shortcut to leave fullscreen via keyboard. */
+export function useFullscreenMode(): UseFullscreenModeResult {
+  const [fullscreen, setFullscreen] = useState(false);
+
+  useEffect(() => {
+    if (fullscreen) document.body.classList.add(BODY_CLASS);
+    else document.body.classList.remove(BODY_CLASS);
+    return () => document.body.classList.remove(BODY_CLASS);
+  }, [fullscreen]);
+
+  // Request/exit browser fullscreen alongside the in-app chrome hide.
+  // Ignore failures (fullscreen requires a user gesture; the chrome-only
+  // mode still works if the API rejects).
+  useEffect(() => {
+    if (fullscreen && !document.fullscreenElement) {
+      document.documentElement.requestFullscreen?.().catch(() => {});
+    } else if (!fullscreen && document.fullscreenElement) {
+      document.exitFullscreen?.().catch(() => {});
+    }
+  }, [fullscreen]);
+
+  // Sync state if the user presses F11/Esc to leave fullscreen from
+  // outside our button.
+  useEffect(() => {
+    const onFsChange = () => {
+      if (!document.fullscreenElement) setFullscreen(false);
+    };
+    document.addEventListener('fullscreenchange', onFsChange);
+    return () => document.removeEventListener('fullscreenchange', onFsChange);
+  }, []);
+
+  useEffect(() => {
+    const onKey = (e: KeyboardEvent) => {
+      if (e.key === 'Escape' && fullscreen) setFullscreen(false);
+    };
+    window.addEventListener('keydown', onKey);
+    return () => window.removeEventListener('keydown', onKey);
+  }, [fullscreen]);
+
+  return {
+    fullscreen,
+    setFullscreen,
+    toggle: () => setFullscreen((f) => !f),
+  };
+}