fix(gallery): stop browser zoom tripping the devtools viewport heuristic

innerHeight is in page CSS pixels and shrinks under browser zoom;
outerHeight does not. At 150-200% zoom a normal window therefore shows an
absolute outer/inner gap of 400-500px, past every threshold, so an
accessibility zoom read as a docked DevTools panel and - at protectionLevel
"maximum" - redirected the guest off the gallery on load. Pre-existing
(the previous threshold was 100px), but the rewrite kept the shape.

The gap is now measured relative to a baseline taken at mount, and the
baseline is re-taken whenever devicePixelRatio changes, which a zoom step
does and a docked panel does not. Only a gap that grows past the threshold
at a constant ratio counts. The mount-time check is dropped: a panel that
is already open at load is indistinguishable from a zoomed window.

frontend/src/hooks/__tests__/useDevToolsProtection.test.tsx

@@ -147,6 +147,66 @@ describe('useDevToolsProtection', () => {
     expect(onDevToolsDetected).toHaveBeenCalledTimes(1);
   });
 
+  describe('viewport heuristic', () => {
+    const setWindowMetrics = (metrics: {
+      innerHeight: number; outerHeight: number; innerWidth: number; outerWidth: number; dpr: number;
+    }) => {
+      for (const [key, value] of Object.entries({
+        innerHeight: metrics.innerHeight,
+        outerHeight: metrics.outerHeight,
+        innerWidth: metrics.innerWidth,
+        outerWidth: metrics.outerWidth,
+        devicePixelRatio: metrics.dpr,
+      })) {
+        Object.defineProperty(window, key, { configurable: true, writable: true, value });
+      }
+    };
+
+    it('does not fire on mount for a window that is merely zoomed', () => {
+      // 200% zoom on a 1000px-tall window: innerHeight halves, outerHeight
+      // does not, so the absolute gap is ~580px — far past every threshold.
+      setWindowMetrics({ innerHeight: 500, outerHeight: 1080, innerWidth: 900, outerWidth: 1000, dpr: 2 });
+      const onDevToolsDetected = vi.fn();
+      renderHook(() => useDevToolsProtection({ enabled: true, onDevToolsDetected, detectionSensitivity: 'high' }));
+      window.dispatchEvent(new Event('resize'));
+
+      expect(onDevToolsDetected).not.toHaveBeenCalled();
+    });
+
+    it('does not fire when the gap grows because the guest zoomed in', () => {
+      setWindowMetrics({ innerHeight: 900, outerHeight: 1000, innerWidth: 1000, outerWidth: 1000, dpr: 1 });
+      const onDevToolsDetected = vi.fn();
+      renderHook(() => useDevToolsProtection({ enabled: true, onDevToolsDetected, detectionSensitivity: 'high' }));
+
+      setWindowMetrics({ innerHeight: 450, outerHeight: 1000, innerWidth: 500, outerWidth: 1000, dpr: 2 });
+      window.dispatchEvent(new Event('resize'));
+
+      expect(onDevToolsDetected).not.toHaveBeenCalled();
+    });
+
+    it('fires when the gap grows at a constant pixel ratio (a docked panel)', () => {
+      setWindowMetrics({ innerHeight: 900, outerHeight: 1000, innerWidth: 1000, outerWidth: 1000, dpr: 1 });
+      const onDevToolsDetected = vi.fn();
+      renderHook(() => useDevToolsProtection({ enabled: true, onDevToolsDetected, detectionSensitivity: 'medium' }));
+
+      setWindowMetrics({ innerHeight: 600, outerHeight: 1000, innerWidth: 1000, outerWidth: 1000, dpr: 1 });
+      window.dispatchEvent(new Event('resize'));
+
+      expect(onDevToolsDetected).toHaveBeenCalledTimes(1);
+    });
+
+    it('ignores a plain window resize, where outer and inner move together', () => {
+      setWindowMetrics({ innerHeight: 900, outerHeight: 1000, innerWidth: 1000, outerWidth: 1000, dpr: 1 });
+      const onDevToolsDetected = vi.fn();
+      renderHook(() => useDevToolsProtection({ enabled: true, onDevToolsDetected, detectionSensitivity: 'high' }));
+
+      setWindowMetrics({ innerHeight: 500, outerHeight: 600, innerWidth: 700, outerWidth: 700, dpr: 1 });
+      window.dispatchEvent(new Event('resize'));
+
+      expect(onDevToolsDetected).not.toHaveBeenCalled();
+    });
+  });
+
   it('lets ordinary keystrokes through', () => {
     renderHook(() => useDevToolsProtection({ enabled: true }));
 

frontend/src/hooks/useDevToolsProtection.ts

@@ -61,9 +61,30 @@ export const useDevToolsProtection = (options: UseDevToolsProtectionOptions) =>
     const threshold = detectionSensitivity === 'high' ? 160 :
                       detectionSensitivity === 'low' ? 260 : 200;
 
+    // The outer/inner gap is measured RELATIVE to a baseline taken when the
+    // hook mounts, not as an absolute number. innerHeight is in page CSS px
+    // (it shrinks under browser zoom) while outerHeight is not, so at 150-200%
+    // zoom the absolute gap on a normal window is 400-500px — an
+    // accessibility zoom looked exactly like a docked panel and, at
+    // protectionLevel "maximum", bounced the guest off the gallery on load.
+    // A zoom step changes devicePixelRatio; docking DevTools does not. So the
+    // baseline is re-taken whenever the ratio changes, and only a gap that
+    // grows past the threshold at a constant ratio counts as DevTools.
+    const measure = () => ({
+      dpr: window.devicePixelRatio || 1,
+      height: window.outerHeight - window.innerHeight,
+      width: window.outerWidth - window.innerWidth,
+    });
+    let baseline = measure();
+
     const detectByWindowSize = () => {
-      if (window.outerHeight - window.innerHeight > threshold ||
-          window.outerWidth - window.innerWidth > threshold) {
+      const now = measure();
+      if (now.dpr !== baseline.dpr) {
+        baseline = now;
+        return;
+      }
+      if (now.height - baseline.height > threshold ||
+          now.width - baseline.width > threshold) {
         handleDevToolsDetected();
       }
     };
@@ -97,10 +118,11 @@ export const useDevToolsProtection = (options: UseDevToolsProtectionOptions) =>
 
     document.addEventListener('contextmenu', handleImageContextMenu);
     document.addEventListener('keydown', handleKeyDown, true);
-    // Docking/undocking DevTools resizes the viewport; the initial call covers
-    // the case where it was already open when the gallery loaded.
+    // Docking/undocking DevTools resizes the viewport. There is deliberately
+    // no check at mount: a panel that was already open when the gallery
+    // loaded is indistinguishable from a zoomed window at that point, and
+    // the shortcut-key deterrent above still applies.
     window.addEventListener('resize', detectByWindowSize);
-    detectByWindowSize();
 
     return () => {
       document.removeEventListener('contextmenu', handleImageContextMenu);