Viewer update and autozoom

frontend/package-lock.json

@@ -10,24 +10,24 @@
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+        "svelte": ">=5 <6",
         "vue": ">=3.2.0"
       }
     },
     "node_modules/@embedpdf/plugin-thumbnail": {
-      "version": "1.3.14",
+      "version": "1.4.1",
-      "resolved": "https://registry.npmjs.org/@embedpdf/plugin-thumbnail/-/plugin-thumbnail-1.3.14.tgz",
+      "resolved": "https://registry.npmjs.org/@embedpdf/plugin-thumbnail/-/plugin-thumbnail-1.4.1.tgz",
-      "integrity": "sha512-cnwb5dG8Jph8XSArys1WFCQ6kK2R5FKoO0B5mDrHFv9Fcm2pKszlmZC/NDoskX4pgNUgSnwhI1X3cP37ebF9Ng==",
+      "integrity": "sha512-bN3msjI0PovazgbPK3LyugYVTwIDo0RyBUhBaG42FgJxeY3hmFOWTPgfUH1QF7twHlySnksIvHRFYR3nViryVw==",
       "license": "MIT",
       "dependencies": {
-        "@embedpdf/models": "1.3.14"
+        "@embedpdf/models": "1.4.1"
       },
       "peerDependencies": {
-        "@embedpdf/core": "1.3.14",
+        "@embedpdf/core": "1.4.1",
-        "@embedpdf/plugin-render": "1.3.14",
+        "@embedpdf/plugin-render": "1.4.1",
         "preact": "^10.26.4",
         "react": ">=16.8.0",
         "react-dom": ">=16.8.0",
+        "svelte": ">=5 <6",
         "vue": ">=3.2.0"
       }
     },
     "node_modules/@embedpdf/plugin-tiling": {
-      "version": "1.3.14",
+      "version": "1.4.1",
-      "resolved": "https://registry.npmjs.org/@embedpdf/plugin-tiling/-/plugin-tiling-1.3.14.tgz",
+      "resolved": "https://registry.npmjs.org/@embedpdf/plugin-tiling/-/plugin-tiling-1.4.1.tgz",
-      "integrity": "sha512-SaCTo2LdZwGeE6jCqkwJxvwt8YKbsI3QGxa9S7Ez+5OcBchlhHeTfLQswcErDQ3WH2p8WHtGuucAcOLrVVOm0A==",
+      "integrity": "sha512-wgTfj5T8HV6KP61iiR63DVNrbVp8sPxTqa1Sm+2/D0jY+EPSSCmpt1/qYWiAXd1X+t78foOjCnfbo7fEMn5/pg==",
       "license": "MIT",
       "dependencies": {
-        "@embedpdf/models": "1.3.14"
+        "@embedpdf/models": "1.4.1"
       },
       "peerDependencies": {
-        "@embedpdf/core": "1.3.14",
+        "@embedpdf/core": "1.4.1",
-        "@embedpdf/plugin-render": "1.3.14",
+        "@embedpdf/plugin-render": "1.4.1",
-        "@embedpdf/plugin-scroll": "1.3.14",
+        "@embedpdf/plugin-scroll": "1.4.1",
-        "@embedpdf/plugin-viewport": "1.3.14",
+        "@embedpdf/plugin-viewport": "1.4.1",
         "preact": "^10.26.4",
         "react": ">=16.8.0",
         "react-dom": ">=16.8.0",
+        "svelte": ">=5 <6",
         "vue": ">=3.2.0"
       }
     },
     "node_modules/@embedpdf/plugin-viewport": {
-      "version": "1.3.14",
+      "version": "1.4.1",
-      "resolved": "https://registry.npmjs.org/@embedpdf/plugin-viewport/-/plugin-viewport-1.3.14.tgz",
+      "resolved": "https://registry.npmjs.org/@embedpdf/plugin-viewport/-/plugin-viewport-1.4.1.tgz",
-      "integrity": "sha512-mfJ7EbbU68eKk6oFvQ4ozGJNpxUxWbjQ5Gm3uuB+Gj5/tWgBocBOX36k/9LgivEEeX7g2S0tOgyErljApmH8Vg==",
+      "integrity": "sha512-+TgFHKPCLTBiDYe2DdsmTS37hwQgcZ3dYIc7bE0l5cp+GVwouu1h0MTmjL+90loizeWwCiu10E/zXR6hz+CUaQ==",
       "license": "MIT",
       "dependencies": {
-        "@embedpdf/models": "1.3.14"
+        "@embedpdf/models": "1.4.1"
       },
       "peerDependencies": {
-        "@embedpdf/core": "1.3.14",
+        "@embedpdf/core": "1.4.1",
         "preact": "^10.26.4",
         "react": ">=16.8.0",
         "react-dom": ">=16.8.0",
+        "svelte": ">=5 <6",
         "vue": ">=3.2.0"
       }
     },
     "node_modules/@embedpdf/plugin-zoom": {
-      "version": "1.3.14",
+      "version": "1.4.1",
-      "resolved": "https://registry.npmjs.org/@embedpdf/plugin-zoom/-/plugin-zoom-1.3.14.tgz",
+      "resolved": "https://registry.npmjs.org/@embedpdf/plugin-zoom/-/plugin-zoom-1.4.1.tgz",
-      "integrity": "sha512-/N5tyMk+8OzhObrS3O9yPkcmX8EPiuTo+WaT2QCVSmIUqKnOO4AnKpHJ6Vl0uVhcuXHCMwLucZKyhJ7tRqavwg==",
+      "integrity": "sha512-9HocmXnPZxqN06q7kyNAmLjgDHOEW8/8QfgNE3nMpRyNHIgnAjxvsWc9lApgp5ErDPG0cSDt0Cduil6nB3wSBQ==",
       "license": "MIT",
       "dependencies": {
-        "@embedpdf/models": "1.3.14",
+        "@embedpdf/models": "1.4.1",
         "hammerjs": "^2.0.8"
       },
       "peerDependencies": {
-        "@embedpdf/core": "1.3.14",
+        "@embedpdf/core": "1.4.1",
-        "@embedpdf/plugin-interaction-manager": "1.3.14",
+        "@embedpdf/plugin-interaction-manager": "1.4.1",
-        "@embedpdf/plugin-scroll": "1.3.14",
+        "@embedpdf/plugin-scroll": "1.4.1",
-        "@embedpdf/plugin-viewport": "1.3.14",
+        "@embedpdf/plugin-viewport": "1.4.1",
         "preact": "^10.26.4",
         "react": ">=16.8.0",
         "react-dom": ">=16.8.0",
+        "svelte": ">=5 <6",
         "vue": ">=3.2.0"
       }
     },
     "node_modules/@embedpdf/utils": {
-      "version": "1.3.14",
+      "version": "1.4.1",
-      "resolved": "https://registry.npmjs.org/@embedpdf/utils/-/utils-1.3.14.tgz",
+      "resolved": "https://registry.npmjs.org/@embedpdf/utils/-/utils-1.4.1.tgz",
-      "integrity": "sha512-gxEJD12nageCMqAjdbicNfDQolXU3nvnV0EX96OdZITRNj0Q1tisutVYoaxcCiJu3vvIEOzipjsAnQOubbFCEA==",
+      "integrity": "sha512-vvJ51Qsz3PyJWR2YvDMMpJXg4+YqdV7Vn2cusmW9sx+4EnAiBiw0HevEE+FepgFV8k+A0WbwXzmsujDIQJ7R4A==",
       "license": "MIT",
       "peerDependencies": {
         "preact": "^10.26.4",
@@ -3045,6 +3061,16 @@
         "url": "https://github.com/sindresorhus/is?sponsor=1"
       }
     },
+    "node_modules/@sveltejs/acorn-typescript": {
+      "version": "1.0.6",
+      "resolved": "https://registry.npmjs.org/@sveltejs/acorn-typescript/-/acorn-typescript-1.0.6.tgz",
+      "integrity": "sha512-4awhxtMh4cx9blePWl10HRHj8Iivtqj+2QdDCSMDzxG+XKa9+VCNupQuCuvzEhYPzZSrX+0gC+0lHA/0fFKKQQ==",
+      "license": "MIT",
+      "peer": true,
+      "peerDependencies": {
+        "acorn": "^8.9.0"
+      }
+    },
     "node_modules/@swc/core": {
       "version": "1.13.5",
       "resolved": "https://registry.npmjs.org/@swc/core/-/core-1.13.5.tgz",
@@ -3802,7 +3828,6 @@
       "version": "1.0.8",
       "resolved": "https://registry.npmjs.org/@types/estree/-/estree-1.0.8.tgz",
       "integrity": "sha512-dWHzHa2WqEXI/O1E9OjrocMTKJl2mSrEolh1Iomrv6U+JuNwaHXsXx9bLu5gG7BUWFIN0skIQJQ/L1rIex4X6w==",
-      "dev": true,
       "license": "MIT"
     },
     "node_modules/@types/gapi": {
@@ -4743,7 +4768,6 @@
       "version": "8.15.0",
       "resolved": "https://registry.npmjs.org/acorn/-/acorn-8.15.0.tgz",
       "integrity": "sha512-NZyJarBfL7nWwIq+FDL6Zp/yHEhePMNnnJ0y3qfieCrmNvYct8uvtiV41UvlSe6apAfk0fY1FbWx+NwfmpvtTg==",
-      "dev": true,
       "license": "MIT",
       "bin": {
         "acorn": "bin/acorn"
@@ -5147,6 +5171,16 @@
         "proxy-from-env": "^1.1.0"
       }
     },
+    "node_modules/axobject-query": {
+      "version": "4.1.0",
+      "resolved": "https://registry.npmjs.org/axobject-query/-/axobject-query-4.1.0.tgz",
+      "integrity": "sha512-qIj0G9wZbMGNLjLmg1PT6v2mE9AH2zlnADJD/2tC6E00hgmhUOfEB6greHPAfLRSufHqROIUTkw6E+M3lH0PTQ==",
+      "license": "Apache-2.0",
+      "peer": true,
+      "engines": {
+        "node": ">= 0.4"
+      }
+    },
     "node_modules/b4a": {
       "version": "1.7.3",
       "resolved": "https://registry.npmjs.org/b4a/-/b4a-1.7.3.tgz",
@@ -7176,6 +7210,13 @@
         "node": "*"
       }
     },
+    "node_modules/esm-env": {
+      "version": "1.2.2",
+      "resolved": "https://registry.npmjs.org/esm-env/-/esm-env-1.2.2.tgz",
+      "integrity": "sha512-Epxrv+Nr/CaL4ZcFGPJIYLWFom+YeV1DqMLHJoEd9SYRxNbaFruBwfEX/kkHUJf55j2+TUbmDcmuilbP1TmXHA==",
+      "license": "MIT",
+      "peer": true
+    },
     "node_modules/espree": {
       "version": "10.4.0",
       "resolved": "https://registry.npmjs.org/espree/-/espree-10.4.0.tgz",
@@ -7234,6 +7275,16 @@
         "node": ">=0.10"
       }
     },
+    "node_modules/esrap": {
+      "version": "2.1.1",
+      "resolved": "https://registry.npmjs.org/esrap/-/esrap-2.1.1.tgz",
+      "integrity": "sha512-ebTT9B6lOtZGMgJ3o5r12wBacHctG7oEWazIda8UlPfA3HD/Wrv8FdXoVo73vzdpwCxNyXjPauyN2bbJzMkB9A==",
+      "license": "MIT",
+      "peer": true,
+      "dependencies": {
+        "@jridgewell/sourcemap-codec": "^1.4.15"
+      }
+    },
     "node_modules/esrecurse": {
       "version": "4.3.0",
       "resolved": "https://registry.npmjs.org/esrecurse/-/esrecurse-4.3.0.tgz",
@@ -8747,6 +8798,16 @@
       "dev": true,
       "license": "MIT"
     },
+    "node_modules/is-reference": {
+      "version": "3.0.3",
+      "resolved": "https://registry.npmjs.org/is-reference/-/is-reference-3.0.3.tgz",
+      "integrity": "sha512-ixkJoqQvAP88E6wLydLGGqCJsrFUnqoH6HnaczB8XmDH1oaWU+xxdptvikTgaEhtZ53Ky6YXiBuUI2WXLMCwjw==",
+      "license": "MIT",
+      "peer": true,
+      "dependencies": {
+        "@types/estree": "^1.0.6"
+      }
+    },
     "node_modules/is-regex": {
       "version": "1.2.1",
       "resolved": "https://registry.npmjs.org/is-regex/-/is-regex-1.2.1.tgz",
@@ -9623,6 +9684,13 @@
         "url": "https://github.com/sponsors/antfu"
       }
     },
+    "node_modules/locate-character": {
+      "version": "3.0.0",
+      "resolved": "https://registry.npmjs.org/locate-character/-/locate-character-3.0.0.tgz",
+      "integrity": "sha512-SW13ws7BjaeJ6p7Q6CO2nchbYEc3X3J6WrmTTDto7yMPqVSZTUyY5Tjbid+Ab8gLnATtygYtiDIJGQRRn2ZOiA==",
+      "license": "MIT",
+      "peer": true
+    },
     "node_modules/locate-path": {
       "version": "6.0.0",
       "resolved": "https://registry.npmjs.org/locate-path/-/locate-path-6.0.0.tgz",
@@ -12959,6 +13027,42 @@
         "url": "https://github.com/sponsors/ljharb"
       }
     },
+    "node_modules/svelte": {
+      "version": "5.43.0",
+      "resolved": "https://registry.npmjs.org/svelte/-/svelte-5.43.0.tgz",
+      "integrity": "sha512-1sRxVbgJAB+UGzwkc3GUoiBSzEOf0jqzccMaVoI2+pI+kASUe9qubslxace8+Mzhqw19k4syTA5niCIJwfXpOA==",
+      "license": "MIT",
+      "peer": true,
+      "dependencies": {
+        "@jridgewell/remapping": "^2.3.4",
+        "@jridgewell/sourcemap-codec": "^1.5.0",
+        "@sveltejs/acorn-typescript": "^1.0.5",
+        "@types/estree": "^1.0.5",
+        "acorn": "^8.12.1",
+        "aria-query": "^5.3.1",
+        "axobject-query": "^4.1.0",
+        "clsx": "^2.1.1",
+        "esm-env": "^1.2.1",
+        "esrap": "^2.1.0",
+        "is-reference": "^3.0.3",
+        "locate-character": "^3.0.0",
+        "magic-string": "^0.30.11",
+        "zimmerframe": "^1.1.2"
+      },
+      "engines": {
+        "node": ">=18"
+      }
+    },
+    "node_modules/svelte/node_modules/aria-query": {
+      "version": "5.3.2",
+      "resolved": "https://registry.npmjs.org/aria-query/-/aria-query-5.3.2.tgz",
+      "integrity": "sha512-COROpnaoap1E2F000S62r6A60uHZnmlvomhfyT2DlTcrY1OrBKn2UhH7qn5wTC9zMvD0AY7csdPSNwKP+7WiQw==",
+      "license": "Apache-2.0",
+      "peer": true,
+      "engines": {
+        "node": ">= 0.4"
+      }
+    },
     "node_modules/symbol-tree": {
       "version": "3.2.4",
       "resolved": "https://registry.npmjs.org/symbol-tree/-/symbol-tree-3.2.4.tgz",
@@ -14447,6 +14551,13 @@
         "url": "https://github.com/sponsors/sindresorhus"
       }
     },
+    "node_modules/zimmerframe": {
+      "version": "1.1.4",
+      "resolved": "https://registry.npmjs.org/zimmerframe/-/zimmerframe-1.1.4.tgz",
+      "integrity": "sha512-B58NGBEoc8Y9MWWCQGl/gq9xBCe4IiKM0a2x7GZdQKOW5Exr8S1W24J6OgM1njK8xCRGvAJIL/MxXHf6SkmQKQ==",
+      "license": "MIT",
+      "peer": true
+    },
     "node_modules/zod": {
       "version": "3.25.76",
       "resolved": "https://registry.npmjs.org/zod/-/zod-3.25.76.tgz",

frontend/package.json

@@ -6,24 +6,24 @@
   "proxy": "http://localhost:8080",
   "dependencies": {
     "@atlaskit/pragmatic-drag-and-drop": "^1.7.7",
-    "@embedpdf/core": "^1.3.14",
+    "@embedpdf/core": "^1.4.1",
-    "@embedpdf/engines": "^1.3.14",
+    "@embedpdf/engines": "^1.4.1",
-    "@embedpdf/plugin-annotation": "^1.3.14",
+    "@embedpdf/plugin-annotation": "^1.4.1",
-    "@embedpdf/plugin-export": "^1.3.14",
+    "@embedpdf/plugin-export": "^1.4.1",
-    "@embedpdf/plugin-history": "^1.3.14",
+    "@embedpdf/plugin-history": "^1.4.1",
-    "@embedpdf/plugin-interaction-manager": "^1.3.14",
+    "@embedpdf/plugin-interaction-manager": "^1.4.1",
-    "@embedpdf/plugin-loader": "^1.3.14",
+    "@embedpdf/plugin-loader": "^1.4.1",
-    "@embedpdf/plugin-pan": "^1.3.14",
+    "@embedpdf/plugin-pan": "^1.4.1",
-    "@embedpdf/plugin-render": "^1.3.14",
+    "@embedpdf/plugin-render": "^1.4.1",
-    "@embedpdf/plugin-rotate": "^1.3.14",
+    "@embedpdf/plugin-rotate": "^1.4.1",
-    "@embedpdf/plugin-scroll": "^1.3.14",
+    "@embedpdf/plugin-scroll": "^1.4.1",
-    "@embedpdf/plugin-search": "^1.3.14",
+    "@embedpdf/plugin-search": "^1.4.1",
-    "@embedpdf/plugin-selection": "^1.3.14",
+    "@embedpdf/plugin-selection": "^1.4.1",
-    "@embedpdf/plugin-spread": "^1.3.14",
+    "@embedpdf/plugin-spread": "^1.4.1",
-    "@embedpdf/plugin-thumbnail": "^1.3.14",
+    "@embedpdf/plugin-thumbnail": "^1.4.1",
-    "@embedpdf/plugin-tiling": "^1.3.14",
+    "@embedpdf/plugin-tiling": "^1.4.1",
-    "@embedpdf/plugin-viewport": "^1.3.14",
+    "@embedpdf/plugin-viewport": "^1.4.1",
-    "@embedpdf/plugin-zoom": "^1.3.14",
+    "@embedpdf/plugin-zoom": "^1.4.1",
     "@emotion/react": "^11.14.0",
     "@emotion/styled": "^11.14.1",
     "@iconify/react": "^6.0.2",

frontend/src/core/components/viewer/EmbedPdfViewer.tsx

@@ -35,14 +35,12 @@ const EmbedPdfViewerContent = ({
   const viewerRef = React.useRef<HTMLDivElement>(null);
   const [isViewerHovered, setIsViewerHovered] = React.useState(false);
 
-  const { isThumbnailSidebarVisible, toggleThumbnailSidebar, zoomActions, spreadActions, panActions: _panActions, rotationActions: _rotationActions, getScrollState, getZoomState, getSpreadState, getRotationState, isAnnotationMode, isAnnotationsVisible, exportActions } = useViewer();
+  const { isThumbnailSidebarVisible, toggleThumbnailSidebar, zoomActions, panActions: _panActions, rotationActions: _rotationActions, getScrollState, getRotationState, isAnnotationMode, isAnnotationsVisible, exportActions } = useViewer();
 
   // Register viewer right-rail buttons
   useViewerRightRailButtons();
 
   const scrollState = getScrollState();
-  const zoomState = getZoomState();
-  const spreadState = getSpreadState();
   const rotationState = getRotationState();
 
   // Track initial rotation to detect changes
@@ -320,15 +318,6 @@ const EmbedPdfViewerContent = ({
             <PdfViewerToolbar
               currentPage={scrollState.currentPage}
               totalPages={scrollState.totalPages}
-              onPageChange={(page) => {
-                // Page navigation handled by scrollActions
-                console.log('Navigate to page:', page);
-              }}
-              dualPage={spreadState.isDualPage}
-              onDualPageToggle={() => {
-                spreadActions.toggleSpreadMode();
-              }}
-              currentZoom={zoomState.zoomPercent}
             />
           </div>
         </div>

frontend/src/core/components/viewer/LocalEmbedPDF.tsx

@@ -8,7 +8,7 @@ import { Viewport, ViewportPluginPackage } from '@embedpdf/plugin-viewport/react
 import { Scroller, ScrollPluginPackage, ScrollStrategy } from '@embedpdf/plugin-scroll/react';
 import { LoaderPluginPackage } from '@embedpdf/plugin-loader/react';
 import { RenderPluginPackage } from '@embedpdf/plugin-render/react';
-import { ZoomPluginPackage } from '@embedpdf/plugin-zoom/react';
+import { ZoomPluginPackage, ZoomMode } from '@embedpdf/plugin-zoom/react';
 import { InteractionManagerPluginPackage, PagePointerProvider, GlobalPointerProvider } from '@embedpdf/plugin-interaction-manager/react';
 import { SelectionLayer, SelectionPluginPackage } from '@embedpdf/plugin-selection/react';
 import { TilingLayer, TilingPluginPackage } from '@embedpdf/plugin-tiling/react';
@@ -114,9 +114,9 @@ export function LocalEmbedPDF({ file, url, enableAnnotations = false, onSignatur
 
       // Register zoom plugin with configuration
       createPluginRegistration(ZoomPluginPackage, {
-        defaultZoomLevel: 1.4, // Start at 140% zoom for better readability
+        defaultZoomLevel: ZoomMode.FitWidth, // Start with FitWidth, will be adjusted in ZoomAPIBridge
         minZoom: 0.2,
-        maxZoom: 3.0,
+        maxZoom: 5.0,
       }),
 
       // Register tiling plugin (depends on Render, Scroll, Viewport)
@@ -287,6 +287,8 @@ export function LocalEmbedPDF({ file, url, enableAnnotations = false, onSignatur
               minHeight: 0,
               minWidth: 0,
               contain: 'strict',
+              display: 'flex',
+              justifyContent: 'center',
             }}
           >
           <Scroller
@@ -295,6 +297,9 @@ export function LocalEmbedPDF({ file, url, enableAnnotations = false, onSignatur
                 <Rotate key={document?.id} pageSize={{ width, height }}>
                   <PagePointerProvider pageIndex={pageIndex} pageWidth={width} pageHeight={height} scale={scale} rotation={rotation}>
                     <div
+                      data-page-index={pageIndex}
+                      data-page-width={width}
+                      data-page-height={height}
                       style={{
                         width,
                         height,

frontend/src/core/components/viewer/PdfViewerToolbar.tsx

@@ -14,30 +14,32 @@ interface PdfViewerToolbarProps {
   currentPage?: number;
   totalPages?: number;
   onPageChange?: (page: number) => void;
-
-  // Dual page toggle (placeholder for now)
-  dualPage?: boolean;
-  onDualPageToggle?: () => void;
-
-  // Zoom controls (connected via ViewerContext)
-  currentZoom?: number;
 }
 
 export function PdfViewerToolbar({
   currentPage = 1,
   totalPages: _totalPages = 1,
   onPageChange,
-  dualPage = false,
-  onDualPageToggle,
-  currentZoom: _currentZoom = 100,
 }: PdfViewerToolbarProps) {
   const { t } = useTranslation();
-  const { getScrollState, getZoomState, scrollActions, zoomActions, registerImmediateZoomUpdate, registerImmediateScrollUpdate } = useViewer();
+  const {
+    getScrollState,
+    getZoomState,
+    getSpreadState,
+    scrollActions,
+    zoomActions,
+    spreadActions,
+    registerImmediateZoomUpdate,
+    registerImmediateScrollUpdate,
+    registerImmediateSpreadUpdate,
+  } = useViewer();
 
   const scrollState = getScrollState();
   const zoomState = getZoomState();
+  const spreadState = getSpreadState();
   const [pageInput, setPageInput] = useState(scrollState.currentPage || currentPage);
   const [displayZoomPercent, setDisplayZoomPercent] = useState(zoomState.zoomPercent || 140);
+  const [isDualPageActive, setIsDualPageActive] = useState(spreadState.isDualPage);
 
   // Register for immediate scroll updates and sync with actual scroll state
   useEffect(() => {
@@ -53,6 +55,13 @@ export function PdfViewerToolbar({
     setDisplayZoomPercent(zoomState.zoomPercent || 140);
   }, [zoomState.zoomPercent, registerImmediateZoomUpdate]);
 
+  useEffect(() => {
+    registerImmediateSpreadUpdate((_mode, isDual) => {
+      setIsDualPageActive(isDual);
+    });
+    setIsDualPageActive(spreadState.isDualPage);
+  }, [registerImmediateSpreadUpdate, spreadState.isDualPage]);
+
   const handleZoomOut = () => {
     zoomActions.zoomOut();
   };
@@ -69,6 +78,10 @@ export function PdfViewerToolbar({
     setPageInput(page);
   };
 
+  const handleDualPageToggle = () => {
+    spreadActions.toggleSpreadMode();
+  };
+
   const handleFirstPage = () => {
     scrollActions.scrollToFirstPage();
   };
@@ -188,15 +201,19 @@ export function PdfViewerToolbar({
 
         {/* Dual Page Toggle */}
         <Button
-          variant={dualPage ? "filled" : "light"}
+          variant={isDualPageActive ? "filled" : "light"}
           color="blue"
           size="md"
           radius="xl"
-          onClick={onDualPageToggle}
+          onClick={handleDualPageToggle}
           style={{ minWidth: '2.5rem' }}
-          title={dualPage ? t("viewer.singlePageView", "Single Page View") : t("viewer.dualPageView", "Dual Page View")}
+          title={
+            isDualPageActive
+              ? t("viewer.singlePageView", "Single Page View")
+              : t("viewer.dualPageView", "Dual Page View")
+          }
         >
-          {dualPage ? <DescriptionIcon fontSize="small" /> : <ViewWeekIcon fontSize="small" />}
+          {isDualPageActive ? <DescriptionIcon fontSize="small" /> : <ViewWeekIcon fontSize="small" />}
         </Button>
 
         {/* Zoom Controls */}

frontend/src/core/components/viewer/SpreadAPIBridge.tsx

@@ -7,16 +7,18 @@ import { useViewer } from '@app/contexts/ViewerContext';
  */
 export function SpreadAPIBridge() {
   const { provides: spread, spreadMode } = useSpread();
-  const { registerBridge } = useViewer();
+  const { registerBridge, triggerImmediateSpreadUpdate } = useViewer();
 
   useEffect(() => {
-    if (spread) {
+    if (!spread) {
+      return;
+    }
+
     const newState = {
       spreadMode,
-        isDualPage: spreadMode !== SpreadMode.None
+      isDualPage: spreadMode !== SpreadMode.None,
     };
 
-      // Register this bridge with ViewerContext
     registerBridge('spread', {
       state: newState,
       api: {
@@ -25,15 +27,16 @@ export function SpreadAPIBridge() {
         },
         getSpreadMode: () => spread.getSpreadMode(),
         toggleSpreadMode: () => {
-            // Toggle between None and Odd (most common dual-page mode)
+          const current = spread.getSpreadMode();
-            const newMode = spreadMode === SpreadMode.None ? SpreadMode.Odd : SpreadMode.None;
+          const nextMode = current === SpreadMode.None ? SpreadMode.Odd : SpreadMode.None;
-            spread.setSpreadMode(newMode);
+          spread.setSpreadMode(nextMode);
+        },
+        SpreadMode,
       },
-          SpreadMode: SpreadMode, // Export enum for reference
-        }
     });
-    }
+
-  }, [spread, spreadMode]);
+    triggerImmediateSpreadUpdate(spreadMode);
+  }, [spread, spreadMode, registerBridge, triggerImmediateSpreadUpdate]);
 
   return null;
 }

frontend/src/core/components/viewer/ZoomAPIBridge.tsx

@@ -1,68 +1,256 @@
-import { useEffect, useRef } from 'react';
+import { useCallback, useEffect, useRef, useState } from 'react';
-import { useZoom } from '@embedpdf/plugin-zoom/react';
+import { useZoom, ZoomMode } from '@embedpdf/plugin-zoom/react';
+import { useSpread, SpreadMode } from '@embedpdf/plugin-spread/react';
 import { useViewer } from '@app/contexts/ViewerContext';
+import { useFileState } from '@app/contexts/FileContext';
+import {
+  determineAutoZoom,
+  DEFAULT_FALLBACK_ZOOM,
+  DEFAULT_VISIBILITY_THRESHOLD,
+  measureRenderedPageRect,
+  useFitWidthResize,
+  ZoomViewport,
+} from '@core/utils/viewerZoom';
+import { getFirstPageAspectRatioFromStub } from '@core/utils/pageMetadata';
 
-/**
- * Component that runs inside EmbedPDF context and manages zoom state locally
- */
 export function ZoomAPIBridge() {
   const { provides: zoom, state: zoomState } = useZoom();
+  const { state: spreadState } = useSpread();
   const { registerBridge, triggerImmediateZoomUpdate } = useViewer();
-  const hasSetInitialZoom = useRef(false);
+  const { selectors } = useFileState();
+
+  const hasSetInitialZoom = useRef(false);
+  const lastSpreadMode = useRef(spreadState?.spreadMode);
+  const lastFileId = useRef<string | undefined>();
+  const lastAppliedZoom = useRef<number | null>(null);
+  const [autoZoomTick, setAutoZoomTick] = useState(0);
+
+  const scheduleAutoZoom = useCallback(() => {
+    hasSetInitialZoom.current = false;
+    lastAppliedZoom.current = null;
+    setAutoZoomTick((tick) => tick + 1);
+  }, []);
+
+  const requestFitWidth = useCallback(() => {
+    if (zoom) {
+      zoom.requestZoom(ZoomMode.FitWidth, { vx: 0.5, vy: 0 });
+    }
+  }, [zoom]);
+
+  const stubs = selectors.getStirlingFileStubs();
+  const firstFileStub = stubs[0];
+  const firstFileId = firstFileStub?.id;
 
-  // Set initial zoom once when plugin is ready
   useEffect(() => {
-    if (!zoom || hasSetInitialZoom.current) {
+    if (!firstFileId) {
+      hasSetInitialZoom.current = false;
+      lastFileId.current = undefined;
+      lastAppliedZoom.current = null;
       return;
     }
 
-    let retryTimer: ReturnType<typeof setTimeout> | undefined;
+    if (firstFileId !== lastFileId.current) {
-    const attemptInitialZoom = () => {
+      lastFileId.current = firstFileId;
-      try {
+      scheduleAutoZoom();
-        zoom.requestZoom(1.4);
-        hasSetInitialZoom.current = true;
-      } catch (error) {
-        console.log('Zoom initialization delayed, viewport not ready:', error);
-        retryTimer = setTimeout(() => {
-          try {
-            zoom.requestZoom(1.4);
-            hasSetInitialZoom.current = true;
-          } catch (retryError) {
-            console.log('Zoom initialization failed:', retryError);
     }
-        }, 200);
+  }, [firstFileId, scheduleAutoZoom]);
-      }
-    };
-
-    const timer = setTimeout(attemptInitialZoom, 50);
-
-    return () => {
-      clearTimeout(timer);
-      if (retryTimer) {
-        clearTimeout(retryTimer);
-      }
-    };
-  }, [zoom, zoomState]);
 
   useEffect(() => {
-    if (zoom && zoomState) {
+    const currentSpreadMode = spreadState?.spreadMode ?? SpreadMode.None;
-      // Update local state
+    if (currentSpreadMode !== lastSpreadMode.current) {
-      const currentZoomLevel = zoomState.currentZoomLevel ?? 1.4;
+      lastSpreadMode.current = currentSpreadMode;
+
+      const hadTrackedAutoZoom = lastAppliedZoom.current !== null;
+      const zoomLevel = zoomState?.zoomLevel;
+      if (
+        zoomLevel === ZoomMode.FitWidth ||
+        zoomLevel === ZoomMode.Automatic ||
+        hadTrackedAutoZoom
+      ) {
+        requestFitWidth();
+        scheduleAutoZoom();
+      }
+    }
+  }, [
+    spreadState?.spreadMode,
+    zoomState?.zoomLevel,
+    scheduleAutoZoom,
+    requestFitWidth,
+  ]);
+
+  const getViewportSnapshot = useCallback((): ZoomViewport | null => {
+    if (!zoomState || typeof zoomState !== 'object') {
+      return null;
+    }
+
+    if ('viewport' in zoomState) {
+      const candidate = (zoomState as { viewport?: ZoomViewport | null }).viewport;
+      return candidate ?? null;
+    }
+
+    return null;
+  }, [zoomState]);
+
+  const isManagedZoom =
+    !!zoom &&
+    (zoomState?.zoomLevel === ZoomMode.FitWidth ||
+      zoomState?.zoomLevel === ZoomMode.Automatic ||
+      lastAppliedZoom.current !== null);
+
+  useFitWidthResize({
+    isManaged: isManagedZoom,
+    requestFitWidth,
+    onDebouncedResize: scheduleAutoZoom,
+  });
+
+  useEffect(() => {
+    if (!zoom || !zoomState) {
+      return;
+    }
+
+    if (!firstFileId) {
+      return;
+    }
+
+    if (hasSetInitialZoom.current) {
+      return;
+    }
+
+    if (zoomState.zoomLevel !== ZoomMode.FitWidth) {
+      if (zoomState.zoomLevel === ZoomMode.Automatic) {
+        requestFitWidth();
+      }
+      return;
+    }
+
+    const fitWidthZoom = zoomState.currentZoomLevel;
+    if (!fitWidthZoom || fitWidthZoom <= 0) {
+      return;
+    }
+
+    const applyTrackedZoom = (level: number | ZoomMode, effectiveZoom: number) => {
+      zoom.requestZoom(level, { vx: 0.5, vy: 0 });
+      lastAppliedZoom.current = effectiveZoom;
+      triggerImmediateZoomUpdate(Math.round(effectiveZoom * 100));
+      hasSetInitialZoom.current = true;
+    };
+
+    let cancelled = false;
+
+    const applyAutoZoom = async () => {
+      const spreadMode = spreadState?.spreadMode ?? SpreadMode.None;
+      const pagesPerSpread = spreadMode !== SpreadMode.None ? 2 : 1;
+      const metadataAspectRatio = getFirstPageAspectRatioFromStub(firstFileStub);
+
+      const viewport = getViewportSnapshot();
+
+      if (cancelled) {
+        return;
+      }
+
+      const metrics = viewport ?? {};
+      const viewportWidth =
+        metrics.clientWidth ?? metrics.width ?? window.innerWidth ?? 0;
+      const viewportHeight =
+        metrics.clientHeight ?? metrics.height ?? window.innerHeight ?? 0;
+
+      if (viewportWidth <= 0 || viewportHeight <= 0) {
+        return;
+      }
+
+      const pageRect = await measureRenderedPageRect({
+        shouldCancel: () => cancelled,
+      });
+      if (cancelled) {
+        return;
+      }
+
+      const decision = determineAutoZoom({
+        viewportWidth,
+        viewportHeight,
+        fitWidthZoom,
+        pagesPerSpread,
+        pageRect: pageRect
+          ? { width: pageRect.width, height: pageRect.height }
+          : undefined,
+        metadataAspectRatio: metadataAspectRatio ?? null,
+        visibilityThreshold: DEFAULT_VISIBILITY_THRESHOLD,
+        fallbackZoom: DEFAULT_FALLBACK_ZOOM,
+      });
+
+      if (decision.type === 'fallback') {
+        applyTrackedZoom(decision.zoom, decision.zoom);
+        return;
+      }
+
+      if (decision.type === 'fitWidth') {
+        applyTrackedZoom(ZoomMode.FitWidth, fitWidthZoom);
+        return;
+      }
+
+      applyTrackedZoom(decision.zoom, decision.zoom);
+    };
+
+    applyAutoZoom();
+
+    return () => {
+      cancelled = true;
+    };
+  }, [
+    zoom,
+    zoomState,
+    firstFileId,
+    firstFileStub,
+    requestFitWidth,
+    getViewportSnapshot,
+    autoZoomTick,
+    spreadState?.spreadMode,
+    triggerImmediateZoomUpdate,
+  ]);
+
+  useEffect(() => {
+    if (!zoom || typeof zoom.onZoomChange !== 'function') {
+      return;
+    }
+
+    const unsubscribe = zoom.onZoomChange((event: { newZoom?: number }) => {
+      if (typeof event?.newZoom !== 'number') {
+        return;
+      }
+      lastAppliedZoom.current = event.newZoom;
+      triggerImmediateZoomUpdate(Math.round(event.newZoom * 100));
+    });
+
+    return () => {
+      if (typeof unsubscribe === 'function') {
+        unsubscribe();
+      }
+    };
+  }, [zoom, triggerImmediateZoomUpdate]);
+
+  useEffect(() => {
+    if (!zoom || !zoomState) {
+      return;
+    }
+
+    const currentZoomLevel =
+      lastAppliedZoom.current ?? zoomState.currentZoomLevel ?? 1;
+
     const newState = {
       currentZoom: currentZoomLevel,
       zoomPercent: Math.round(currentZoomLevel * 100),
     };
 
-      // Trigger immediate update for responsive UI
     triggerImmediateZoomUpdate(newState.zoomPercent);
 
-      // Register this bridge with ViewerContext
     registerBridge('zoom', {
       state: newState,
-        api: zoom
+      api: zoom,
     });
-    }
+  }, [zoom, zoomState, registerBridge, triggerImmediateZoomUpdate]);
-  }, [zoom, zoomState]);
 
   return null;
 }
+
+
+

frontend/src/core/contexts/ViewerContext.tsx

@@ -1,4 +1,11 @@
-import React, { createContext, useContext, useState, ReactNode, useRef } from 'react';
+import React, {
+  createContext,
+  useContext,
+  useState,
+  ReactNode,
+  useRef,
+  useCallback,
+} from 'react';
 import { SpreadMode } from '@embedpdf/plugin-spread/react';
 import { useNavigation } from '@app/contexts/NavigationContext';
 
@@ -109,6 +116,20 @@ interface BridgeRef<TState = unknown, TApi = unknown> {
   api: TApi;
 }
 
+function useImmediateNotifier<Args extends unknown[]>() {
+  const callbackRef = useRef<((...args: Args) => void) | null>(null);
+
+  const register = useCallback((callback: (...args: Args) => void) => {
+    callbackRef.current = callback;
+  }, []);
+
+  const trigger = useCallback((...args: Args) => {
+    callbackRef.current?.(...args);
+  }, []);
+
+  return { register, trigger };
+}
+
 /**
  * ViewerContext provides a unified interface to EmbedPDF functionality.
  *
@@ -150,10 +171,12 @@ interface ViewerContextType {
   // Immediate update callbacks
   registerImmediateZoomUpdate: (callback: (percent: number) => void) => void;
   registerImmediateScrollUpdate: (callback: (currentPage: number, totalPages: number) => void) => void;
+  registerImmediateSpreadUpdate: (callback: (mode: SpreadMode, isDualPage: boolean) => void) => void;
 
   // Internal - for bridges to trigger immediate updates
   triggerImmediateScrollUpdate: (currentPage: number, totalPages: number) => void;
   triggerImmediateZoomUpdate: (zoomPercent: number) => void;
+  triggerImmediateSpreadUpdate: (mode: SpreadMode, isDualPage?: boolean) => void;
 
   // Action handlers - call EmbedPDF APIs directly
   scrollActions: {
@@ -241,11 +264,39 @@ export const ViewerProvider: React.FC<ViewerProviderProps> = ({ children }) => {
     export: null as BridgeRef<ExportState, ExportAPIWrapper> | null,
   });
 
-  // Immediate zoom callback for responsive display updates
+  const {
-  const immediateZoomUpdateCallback = useRef<((percent: number) => void) | null>(null);
+    register: registerImmediateZoomUpdate,
+    trigger: triggerImmediateZoomInternal,
+  } = useImmediateNotifier<[number]>();
+  const {
+    register: registerImmediateScrollUpdate,
+    trigger: triggerImmediateScrollInternal,
+  } = useImmediateNotifier<[number, number]>();
+  const {
+    register: registerImmediateSpreadUpdate,
+    trigger: triggerImmediateSpreadInternal,
+  } = useImmediateNotifier<[SpreadMode, boolean]>();
 
-  // Immediate scroll callback for responsive display updates
+  const triggerImmediateZoomUpdate = useCallback(
-  const immediateScrollUpdateCallback = useRef<((currentPage: number, totalPages: number) => void) | null>(null);
+    (percent: number) => {
+      triggerImmediateZoomInternal(percent);
+    },
+    [triggerImmediateZoomInternal]
+  );
+
+  const triggerImmediateScrollUpdate = useCallback(
+    (currentPage: number, totalPages: number) => {
+      triggerImmediateScrollInternal(currentPage, totalPages);
+    },
+    [triggerImmediateScrollInternal]
+  );
+
+  const triggerImmediateSpreadUpdate = useCallback(
+    (mode: SpreadMode, isDualPage: boolean = mode !== SpreadMode.None) => {
+      triggerImmediateSpreadInternal(mode, isDualPage);
+    },
+    [triggerImmediateSpreadInternal]
+  );
 
   const registerBridge = (type: string, ref: BridgeRef) => {
     // Type-safe assignment - we know the bridges will provide correct types
@@ -372,24 +423,18 @@ export const ViewerProvider: React.FC<ViewerProviderProps> = ({ children }) => {
     zoomIn: () => {
       const api = bridgeRefs.current.zoom?.api;
       if (api?.zoomIn) {
-        // Update display immediately if callback is registered
-        if (immediateZoomUpdateCallback.current) {
         const currentState = getZoomState();
         const newPercent = Math.min(Math.round(currentState.zoomPercent * 1.2), 300);
-          immediateZoomUpdateCallback.current(newPercent);
+        triggerImmediateZoomUpdate(newPercent);
-        }
         api.zoomIn();
       }
     },
     zoomOut: () => {
       const api = bridgeRefs.current.zoom?.api;
       if (api?.zoomOut) {
-        // Update display immediately if callback is registered
-        if (immediateZoomUpdateCallback.current) {
         const currentState = getZoomState();
         const newPercent = Math.max(Math.round(currentState.zoomPercent / 1.2), 20);
-          immediateZoomUpdateCallback.current(newPercent);
+        triggerImmediateZoomUpdate(newPercent);
-        }
         api.zoomOut();
       }
     },
@@ -550,26 +595,6 @@ export const ViewerProvider: React.FC<ViewerProviderProps> = ({ children }) => {
     }
   };
 
-  const registerImmediateZoomUpdate = (callback: (percent: number) => void) => {
-    immediateZoomUpdateCallback.current = callback;
-  };
-
-  const registerImmediateScrollUpdate = (callback: (currentPage: number, totalPages: number) => void) => {
-    immediateScrollUpdateCallback.current = callback;
-  };
-
-  const triggerImmediateScrollUpdate = (currentPage: number, totalPages: number) => {
-    if (immediateScrollUpdateCallback.current) {
-      immediateScrollUpdateCallback.current(currentPage, totalPages);
-    }
-  };
-
-  const triggerImmediateZoomUpdate = (zoomPercent: number) => {
-    if (immediateZoomUpdateCallback.current) {
-      immediateZoomUpdateCallback.current(zoomPercent);
-    }
-  };
-
   const value: ViewerContextType = {
     // UI state
     isThumbnailSidebarVisible,
@@ -600,8 +625,10 @@ export const ViewerProvider: React.FC<ViewerProviderProps> = ({ children }) => {
     // Immediate updates
     registerImmediateZoomUpdate,
     registerImmediateScrollUpdate,
+    registerImmediateSpreadUpdate,
     triggerImmediateScrollUpdate,
     triggerImmediateZoomUpdate,
+    triggerImmediateSpreadUpdate,
 
     // Actions
     scrollActions,

frontend/src/core/contexts/file/fileActions.ts

@@ -58,14 +58,21 @@ const addFilesMutex = new SimpleMutex();
 /**
  * Helper to create ProcessedFile metadata structure
  */
-export function createProcessedFile(pageCount: number, thumbnail?: string, pageRotations?: number[]) {
+export function createProcessedFile(
+  pageCount: number,
+  thumbnail?: string,
+  pageRotations?: number[],
+  pageDimensions?: Array<{ width: number; height: number }>
+) {
   return {
     totalPages: pageCount,
     pages: Array.from({ length: pageCount }, (_, index) => ({
       pageNumber: index + 1,
       thumbnail: index === 0 ? thumbnail : undefined, // Only first page gets thumbnail initially
       rotation: pageRotations?.[index] ?? 0,
-      splitBefore: false
+      splitBefore: false,
+      width: pageDimensions?.[index]?.width,
+      height: pageDimensions?.[index]?.height
     })),
     thumbnailUrl: thumbnail,
     lastProcessed: Date.now()
@@ -92,7 +99,8 @@ export async function generateProcessedFileMetadata(file: File): Promise<Process
     const processedFile = createProcessedFile(
       unrotatedResult.pageCount,
       unrotatedResult.thumbnail, // Page thumbnails (unrotated)
-      unrotatedResult.pageRotations
+      unrotatedResult.pageRotations,
+      unrotatedResult.pageDimensions
     );
 
     // Use rotated thumbnail for file manager

frontend/src/core/types/fileContext.ts

@@ -14,6 +14,8 @@ export interface ProcessedFilePage {
   pageNumber?: number;
   rotation?: number;
   splitBefore?: boolean;
+  width?: number;
+  height?: number;
   [key: string]: any;
 }
 

frontend/src/core/utils/pageMetadata.ts

@@ -0,0 +1,53 @@
+import {
+  ProcessedFileMetadata,
+  ProcessedFilePage,
+  StirlingFileStub,
+} from '@app/types/fileContext';
+
+export interface PageDimensions {
+  width: number | null;
+  height: number | null;
+}
+
+export function getPageDimensions(
+  page?: ProcessedFilePage | null
+): PageDimensions {
+  const width =
+    typeof page?.width === 'number' && page.width > 0 ? page.width : null;
+  const height =
+    typeof page?.height === 'number' && page.height > 0 ? page.height : null;
+
+  return { width, height };
+}
+
+export function getFirstPageDimensionsFromMetadata(
+  metadata?: ProcessedFileMetadata | null
+): PageDimensions {
+  if (!metadata?.pages?.length) {
+    return { width: null, height: null };
+  }
+
+  return getPageDimensions(metadata.pages[0]);
+}
+
+export function getFirstPageDimensionsFromStub(
+  file?: StirlingFileStub
+): PageDimensions {
+  return getFirstPageDimensionsFromMetadata(file?.processedFile);
+}
+
+export function getFirstPageAspectRatioFromMetadata(
+  metadata?: ProcessedFileMetadata | null
+): number | null {
+  const { width, height } = getFirstPageDimensionsFromMetadata(metadata);
+  if (width && height) {
+    return height / width;
+  }
+  return null;
+}
+
+export function getFirstPageAspectRatioFromStub(
+  file?: StirlingFileStub
+): number | null {
+  return getFirstPageAspectRatioFromMetadata(file?.processedFile);
+}

frontend/src/core/utils/thumbnailUtils.ts

@@ -4,6 +4,7 @@ export interface ThumbnailWithMetadata {
   thumbnail: string; // Always returns a thumbnail (placeholder if needed)
   pageCount: number;
   pageRotations?: number[]; // Rotation for each page (0, 90, 180, 270)
+  pageDimensions?: Array<{ width: number; height: number }>;
 }
 
 interface ColorScheme {
@@ -402,12 +403,18 @@ export async function generateThumbnailWithMetadata(file: File, applyRotation: b
 
     const pageCount = pdf.numPages;
     const page = await pdf.getPage(1);
+    const pageDimensions: Array<{ width: number; height: number }> = [];
 
     // If applyRotation is false, render without rotation (for CSS-based rotation)
     // If applyRotation is true, let PDF.js apply rotation (for static display)
     const viewport = applyRotation
       ? page.getViewport({ scale })
       : page.getViewport({ scale, rotation: 0 });
+    const baseViewport = page.getViewport({ scale: 1, rotation: 0 });
+    pageDimensions[0] = {
+      width: baseViewport.width,
+      height: baseViewport.height
+    };
 
     const canvas = document.createElement("canvas");
     canvas.width = viewport.width;
@@ -428,10 +435,17 @@ export async function generateThumbnailWithMetadata(file: File, applyRotation: b
       const p = await pdf.getPage(i);
       const rotation = p.rotate || 0;
       pageRotations.push(rotation);
+      if (!pageDimensions[i - 1]) {
+        const pageViewport = p.getViewport({ scale: 1, rotation: 0 });
+        pageDimensions[i - 1] = {
+          width: pageViewport.width,
+          height: pageViewport.height
+        };
+      }
     }
 
     pdfWorkerManager.destroyDocument(pdf);
-    return { thumbnail, pageCount, pageRotations };
+    return { thumbnail, pageCount, pageRotations, pageDimensions };
 
   } catch (error) {
     if (error instanceof Error && error.name === "PasswordException") {

frontend/src/core/utils/viewerZoom.ts

@@ -0,0 +1,188 @@
+import { useEffect, useRef } from 'react';
+
+export const DEFAULT_VISIBILITY_THRESHOLD = 80; // Require at least 80% of the page height to be visible
+export const DEFAULT_FALLBACK_ZOOM = 1.44; // 144% fallback when no reliable metadata is present
+
+export interface ZoomViewport {
+  clientWidth?: number;
+  clientHeight?: number;
+  width?: number;
+  height?: number;
+}
+
+export type AutoZoomDecision =
+  | { type: 'fallback'; zoom: number }
+  | { type: 'fitWidth' }
+  | { type: 'adjust'; zoom: number };
+
+export interface AutoZoomParams {
+  viewportWidth: number;
+  viewportHeight: number;
+  fitWidthZoom: number;
+  pagesPerSpread: number;
+  pageRect?: { width: number; height: number } | null;
+  metadataAspectRatio?: number | null;
+  visibilityThreshold?: number;
+  fallbackZoom?: number;
+}
+
+export function determineAutoZoom({
+  viewportWidth,
+  viewportHeight,
+  fitWidthZoom,
+  pagesPerSpread,
+  pageRect,
+  metadataAspectRatio,
+  visibilityThreshold = DEFAULT_VISIBILITY_THRESHOLD,
+  fallbackZoom = DEFAULT_FALLBACK_ZOOM,
+}: AutoZoomParams): AutoZoomDecision {
+  const rectWidth = pageRect?.width ?? 0;
+  const rectHeight = pageRect?.height ?? 0;
+
+  const aspectRatio: number | null =
+    rectWidth > 0 ? rectHeight / rectWidth : metadataAspectRatio ?? null;
+
+  let renderedHeight: number | null = rectHeight > 0 ? rectHeight : null;
+
+  if (!renderedHeight || renderedHeight <= 0) {
+    if (aspectRatio == null || aspectRatio <= 0) {
+      return { type: 'fallback', zoom: Math.min(fitWidthZoom, fallbackZoom) };
+    }
+
+    const pageWidth = viewportWidth / (fitWidthZoom * pagesPerSpread);
+    const pageHeight = pageWidth * aspectRatio;
+    renderedHeight = pageHeight * fitWidthZoom;
+  }
+
+  if (!renderedHeight || renderedHeight <= 0) {
+    return { type: 'fitWidth' };
+  }
+
+  const isLandscape = aspectRatio !== null && aspectRatio < 1;
+  const targetVisibility = isLandscape ? 100 : visibilityThreshold;
+
+  const visiblePercent = (viewportHeight / renderedHeight) * 100;
+
+  if (visiblePercent >= targetVisibility) {
+    return { type: 'fitWidth' };
+  }
+
+  const allowableHeightRatio = targetVisibility / 100;
+  const zoomScale =
+    viewportHeight / (allowableHeightRatio * renderedHeight);
+  const targetZoom = Math.min(fitWidthZoom, fitWidthZoom * zoomScale);
+
+  if (Math.abs(targetZoom - fitWidthZoom) < 0.001) {
+    return { type: 'fitWidth' };
+  }
+
+  return { type: 'adjust', zoom: targetZoom };
+}
+
+export interface MeasurePageRectOptions {
+  selector?: string;
+  maxAttempts?: number;
+  shouldCancel?: () => boolean;
+}
+
+export async function measureRenderedPageRect({
+  selector = '[data-page-index="0"]',
+  maxAttempts = 12,
+  shouldCancel,
+}: MeasurePageRectOptions = {}): Promise<DOMRect | null> {
+  if (typeof window === 'undefined' || typeof document === 'undefined') {
+    return null;
+  }
+
+  let rafId: number | null = null;
+
+  const waitForNextFrame = () =>
+    new Promise<void>((resolve) => {
+      rafId = window.requestAnimationFrame(() => {
+        rafId = null;
+        resolve();
+      });
+    });
+
+  try {
+    for (let attempt = 0; attempt < maxAttempts; attempt++) {
+      if (shouldCancel?.()) {
+        return null;
+      }
+
+      const element = document.querySelector(selector) as HTMLElement | null;
+
+      if (element) {
+        const rect = element.getBoundingClientRect();
+        if (rect.width > 0 && rect.height > 0) {
+          return rect;
+        }
+      }
+
+      await waitForNextFrame();
+    }
+  } finally {
+    if (rafId !== null) {
+      window.cancelAnimationFrame(rafId);
+    }
+  }
+
+  return null;
+}
+
+export interface FitWidthResizeOptions {
+  isManaged: boolean;
+  requestFitWidth: () => void;
+  onDebouncedResize: () => void;
+  debounceMs?: number;
+}
+
+export function useFitWidthResize({
+  isManaged,
+  requestFitWidth,
+  onDebouncedResize,
+  debounceMs = 150,
+}: FitWidthResizeOptions): void {
+  const managedRef = useRef(isManaged);
+  const requestFitWidthRef = useRef(requestFitWidth);
+  const onDebouncedResizeRef = useRef(onDebouncedResize);
+
+  useEffect(() => {
+    managedRef.current = isManaged;
+  }, [isManaged]);
+
+  useEffect(() => {
+    requestFitWidthRef.current = requestFitWidth;
+  }, [requestFitWidth]);
+
+  useEffect(() => {
+    onDebouncedResizeRef.current = onDebouncedResize;
+  }, [onDebouncedResize]);
+
+  useEffect(() => {
+    let timeoutId: number | undefined;
+
+    const handleResize = () => {
+      if (!managedRef.current) {
+        return;
+      }
+
+      if (typeof timeoutId === 'number') {
+        window.clearTimeout(timeoutId);
+      }
+
+      timeoutId = window.setTimeout(() => {
+        requestFitWidthRef.current?.();
+        onDebouncedResizeRef.current?.();
+      }, debounceMs);
+    };
+
+    window.addEventListener('resize', handleResize);
+    return () => {
+      if (typeof timeoutId === 'number') {
+        window.clearTimeout(timeoutId);
+      }
+      window.removeEventListener('resize', handleResize);
+    };
+  }, [debounceMs]);
+}