fix: eliminate image flicker by gating opacity on fit-transform readiness

frontend/src/components/Media/ZoomableImage.tsx

@@ -39,6 +39,7 @@ export const ZoomableImage = forwardRef<ZoomableImageRef, ZoomableImageProps>(
       zoomIn,
       zoomOut,
       reset,
+      isFitReady
     } = useZoomTransform({ imagePath, rotation, resetSignal });
 
     useImperativeHandle(
@@ -113,6 +114,8 @@ export const ZoomableImage = forwardRef<ZoomableImageRef, ZoomableImageProps>(
               transform: `rotate(${rotation}deg)`,
               transformOrigin: 'center center',
               pointerEvents: 'none',
+              opacity: isFitReady ? 1 : 0,
+              transition: isFitReady ? 'opacity 120ms ease-out' : 'none',
             }}
           />
         </div>

frontend/src/components/Media/__tests__/ZoomableImage.test.tsx

@@ -697,6 +697,28 @@ describe('ZoomableImage controlled transform behavior', () => {
     expect(image.style.height).toBe('');
   });
 
+  test('keeps the image hidden until the fit transform is ready, then fades it in', () => {
+    renderZoomableImage();
+
+    const viewport = screen.getByTestId('zoom-viewport');
+    const image = screen.getByAltText('test image');
+
+    expect(image.style.opacity).toBe('0');
+    expect(image.style.transition).toBe('none');
+
+    mockElementRect(
+      viewport,
+      { width: 500, height: 400, left: 0, top: 0 },
+      { clientWidth: 500, clientHeight: 400 },
+    );
+    mockImageDimensions(image, { width: 1000, height: 800 });
+
+    fireEvent.load(image);
+
+    expect(image.style.opacity).toBe('1');
+    expect(image.style.transition).toBe('opacity 120ms ease-out');
+  });
+
   test('starts from the new image fit transform after switching images', () => {
     const { viewport, rerender } = setupScene({
       viewportSize: { width: 500, height: 400 },

frontend/src/hooks/useZoomTransform.ts

@@ -71,6 +71,7 @@ export const useZoomTransform = ({
   const [isOverflowing, setIsOverflowing] = useState(false);
   const [isPanning, setIsPanning] = useState(false);
   const [isButtonZoom, setIsButtonZoom] = useState(false);
+  const [isFitReady, setIsFitReady] = useState(false);
 
   const setRawImageDimensions = useCallback((dimensions: Size | null) => {
     rawDimensionsRef.current = dimensions;
@@ -196,6 +197,7 @@ export const useZoomTransform = ({
       isFitInitializedRef.current = true;
       hasUserInteractedRef.current = false;
       fitRetryCountRef.current = 0;
+      setIsFitReady(true);
     }
 
     return didApply;
@@ -297,6 +299,7 @@ export const useZoomTransform = ({
   useEffect(() => {
     setIsOverflowing(false);
     setRawImageDimensions(null);
+    setIsFitReady(false);
     resetToFit();
   }, [imagePath, resetToFit, setRawImageDimensions]);
 
@@ -556,5 +559,6 @@ export const useZoomTransform = ({
     zoomIn,
     zoomOut,
     reset: resetToFit,
+    isFitReady,
   };
 };