queue

frontend/src/app/AppLayout.tsx

@@ -37,16 +37,12 @@ const AppLayout: React.FC = () => {
   });
 
   if (['logged-out', 'authenticating', 'selecting-tenant'].includes(appStatus)) {
-    const shouldRememberLastLocation = appStatus !== 'logged-out';
+    const redirectTarget = `${location.pathname}${location.search}${location.hash || ''}`;
     return (
       <Navigate
         to="/account/login"
         replace
-        state={
-          shouldRememberLastLocation
-            ? { from: location.pathname + location.search }
-            : undefined
-        }
+        state={{ from: redirectTarget }}
       />
     );
   }

frontend/src/preview/PdfViewer.tsx

@@ -39,6 +39,12 @@ interface PageDescriptor {
   scale: number;
 }
 
+interface RenderQueueRequest {
+  pageNumber: number;
+  resume: () => void;
+  cancel: () => void;
+}
+
 const RERENDER_DELTA = 48;
 
 const getAvailableViewportSize = (viewportNode: Element, stackNode: Element) => {
@@ -85,6 +91,50 @@ const PdfViewer = ({ src, title, className, viewportRef }: PdfViewerProps): JSX.
   const wasmUrlRef = useRef<string | null>(null);
   const pageNodeMapRef = useRef<Map<number, HTMLElement>>(new Map());
   const intersectionObserverRef = useRef<IntersectionObserver | null>(null);
+  const renderQueueRef = useRef<RenderQueueRequest[]>([]);
+  const activeRendersRef = useRef(new Set<number>());
+  const maxConcurrentRendersRef = useRef(2);
+
+  const requestQueueFlush = useCallback(() => {
+    const queue = renderQueueRef.current;
+    while (queue.length > 0 && activeRendersRef.current.size < maxConcurrentRendersRef.current) {
+      const next = queue.pop();
+      if (!next) {
+        break;
+      }
+      if (activeRendersRef.current.has(next.pageNumber)) {
+        next.cancel();
+        continue;
+      }
+      activeRendersRef.current.add(next.pageNumber);
+      next.resume();
+    }
+  }, []);
+
+  const enqueueRender = useCallback((request: RenderQueueRequest) => {
+    renderQueueRef.current.push(request);
+    requestQueueFlush();
+  }, [requestQueueFlush]);
+
+  const releaseRenderSlot = useCallback((pageNumber: number) => {
+    if (activeRendersRef.current.delete(pageNumber)) {
+      requestQueueFlush();
+    }
+  }, [requestQueueFlush]);
+
+  const cancelRenderRequest = useCallback((pageNumber: number) => {
+    const queue = renderQueueRef.current;
+    const index = queue.findIndex((entry) => entry.pageNumber === pageNumber);
+    if (index >= 0) {
+      const [entry] = queue.splice(index, 1);
+      entry.cancel();
+    }
+    releaseRenderSlot(pageNumber);
+  }, [releaseRenderSlot]);
+
+  const handleRenderFinished = useCallback((pageNumber: number) => {
+    releaseRenderSlot(pageNumber);
+  }, [releaseRenderSlot]);
   const focusTargetRef = useRef<{
     ratioX: number;
     ratioY: number;
@@ -428,11 +478,14 @@ const PdfViewer = ({ src, title, className, viewportRef }: PdfViewerProps): JSX.
           viewMode={viewMode}
           shouldRender={shouldRenderPage(page.number)}
           registerPageNode={registerPageNode}
+          enqueueRender={enqueueRender}
+          cancelRenderRequest={cancelRenderRequest}
+          onRenderFinished={handleRenderFinished}
         />
       </Fragment>
     ))
-  ), [handlePageClick, pages, pixelRatio, registerPageNode, shouldRenderPage,
-    viewportHeight, viewportWidth, viewMode]);
+  ), [cancelRenderRequest, enqueueRender, handlePageClick, handleRenderFinished,
+    pages, pixelRatio, registerPageNode, shouldRenderPage, viewportHeight, viewportWidth, viewMode]);
 
   return (
     <div className={viewerClasses}>
@@ -466,6 +519,9 @@ interface PdfPageCanvasProps {
   viewMode: ViewMode;
   shouldRender: boolean;
   registerPageNode: (pageNumber: number, node: HTMLElement | null) => void;
+  enqueueRender: (request: RenderQueueRequest) => void;
+  cancelRenderRequest: (pageNumber: number) => void;
+  onRenderFinished: (pageNumber: number) => void;
 }
 
 function PdfPageCanvas({
@@ -478,6 +534,9 @@ function PdfPageCanvas({
   viewMode,
   shouldRender,
   registerPageNode,
+  enqueueRender,
+  cancelRenderRequest,
+  onRenderFinished,
 }: PdfPageCanvasProps): JSX.Element {
   const wrapperRef = useRef<HTMLDivElement | null>(null);
   const canvasRef = useRef<HTMLCanvasElement>(null);
@@ -488,6 +547,18 @@ function PdfPageCanvas({
     registerPageNode(descriptor.number, node);
   }, [descriptor.number, registerPageNode]);
   const [canvasMounted, setCanvasMounted] = useState(false);
+  const [allowRender, setAllowRender] = useState(false);
+
+  const resetCanvas = useCallback(() => {
+    const canvas = canvasRef.current;
+    if (!canvas) {
+      return;
+    }
+    canvas.width = 0;
+    canvas.height = 0;
+    const context = canvas.getContext('2d');
+    context?.clearRect(0, 0, context.canvas.width || 0, context.canvas.height || 0);
+  }, []);
 
   const renderPage = useCallback(async () => {
     if (renderTaskRef.current || hasRenderedRef.current) {
@@ -496,51 +567,31 @@ function PdfPageCanvas({
     const pdf = pdfRef.current;
     const canvas = canvasRef.current;
     if (!pdf || !canvas) {
+      onRenderFinished(descriptor.number);
       return;
     }
     try {
       const page = await pdf.getPage(descriptor.number);
       const viewport = page.getViewport({ scale: descriptor.scale });
       const qualityScale = 1;
-      const supportsOffscreen = typeof OffscreenCanvas !== 'undefined';
-      let renderContext: CanvasRenderingContext2D | OffscreenCanvasRenderingContext2D | null = null;
-      let offscreenCanvas: OffscreenCanvas | null = null;
-
-      if (supportsOffscreen) {
-        offscreenCanvas = new OffscreenCanvas(
-          viewport.width * pixelRatio * qualityScale,
-          viewport.height * pixelRatio * qualityScale,
-        );
-        renderContext = offscreenCanvas.getContext('2d');
-      } else {
-        renderContext = canvas.getContext('2d');
-        if (!renderContext) {
-          return;
-        }
-        canvas.width = viewport.width * pixelRatio;
-        canvas.height = viewport.height * pixelRatio;
+      const renderContext = canvas.getContext('2d');
+      if (!renderContext) {
+        return;
       }
 
-      renderContext?.setTransform(pixelRatio * qualityScale, 0, 0, pixelRatio * qualityScale, 0, 0);
+      canvas.width = viewport.width * pixelRatio * qualityScale;
+      canvas.height = viewport.height * pixelRatio * qualityScale;
+      renderContext.setTransform(pixelRatio * qualityScale, 0, 0, pixelRatio * qualityScale, 0, 0);
 
       const renderTask = page.render({
-        canvasContext: renderContext as CanvasRenderingContext2D,
+        canvasContext: renderContext,
         viewport,
-        canvas: supportsOffscreen ? undefined : canvas,
+        canvas,
       });
       renderTaskRef.current = renderTask;
       await renderTask.promise;
       hasRenderedRef.current = true;
 
-      if (offscreenCanvas) {
-        const bitmap = offscreenCanvas.transferToImageBitmap();
-        canvas.width = bitmap.width;
-        canvas.height = bitmap.height;
-        const destContext = canvas.getContext('2d');
-        destContext?.drawImage(bitmap, 0, 0);
-        bitmap.close();
-      }
-
       page.cleanup();
 
       const wrapper = wrapperRef.current;
@@ -554,8 +605,9 @@ function PdfPageCanvas({
       console.error('Failed to render PDF page', descriptor.number, error);
     } finally {
       renderTaskRef.current = null;
+      onRenderFinished(descriptor.number);
     }
-  }, [descriptor.number, descriptor.scale, pdfRef, pixelRatio]);
+  }, [descriptor.number, descriptor.scale, onRenderFinished, pdfRef, pixelRatio]);
 
   useEffect(() => {
     if (!canvasMounted) {
@@ -579,10 +631,39 @@ function PdfPageCanvas({
     setCanvasMounted(false);
     hasRenderedRef.current = false;
     renderTaskRef.current?.cancel();
-  }, [descriptor.number]);
+    resetCanvas();
+  }, [descriptor.number, resetCanvas]);
 
   useEffect(() => {
-    if (shouldRender) {
+    if (!shouldRender) {
+      setAllowRender(false);
+      cancelRenderRequest(descriptor.number);
+      return;
+    }
+    let cancelled = false;
+    const request: RenderQueueRequest = {
+      pageNumber: descriptor.number,
+      resume: () => {
+        if (cancelled) {
+          return;
+        }
+        setAllowRender(true);
+      },
+      cancel: () => {
+        cancelled = true;
+        setAllowRender(false);
+      },
+    };
+    enqueueRender(request);
+    return () => {
+      cancelled = true;
+      setAllowRender(false);
+      cancelRenderRequest(descriptor.number);
+    };
+  }, [cancelRenderRequest, descriptor.number, enqueueRender, shouldRender]);
+
+  useEffect(() => {
+    if (allowRender) {
       setCanvasMounted((prev) => (prev ? prev : true));
       return;
     }
@@ -593,16 +674,17 @@ function PdfPageCanvas({
       hasRenderedRef.current = false;
       renderTaskRef.current?.cancel();
       wrapperRef.current?.querySelector('.textLayer')?.remove();
+      resetCanvas();
       return false;
     });
-  }, [shouldRender]);
+  }, [allowRender, resetCanvas]);
 
   useEffect(() => {
-    if (!canvasMounted) {
+    if (!canvasMounted || !allowRender) {
       return;
     }
     void renderPage();
-  }, [canvasMounted, renderPage]);
+  }, [allowRender, canvasMounted, renderPage]);
 
   const limitAxis: 'width' | 'height' = useMemo(() => {
     if (viewMode === 'fit-width') {