merge files inital commit

src/App.tsx

@@ -9,6 +9,7 @@ import ActionDialog, {
   type ActionDialogAction,
 } from './components/ActionDialog';
 import HelpDialog from './components/HelpDialog';
+import MergeQueuePanel from './components/MergeQueuePanel';
 import { PDFDocument } from 'pdf-lib';
 import type {
   StoredWorkspace,
@@ -32,7 +33,7 @@ import {
 import type { PageRef, PdfFile } from './pdf/pdfTypes';
 import {
   loadPdfFromFile,
-  mergePdfFiles,
+  mergePdfFilesAtPosition,
   splitIntoSinglePages,
   exportPages,
 } from './pdf/pdfService';
@@ -47,6 +48,13 @@ import {
   createSelectionWorkspaceName,
   getSelectedPagesInVisualOrder,
 } from './workspace/workspaceSelection';
+import { useMergeQueue } from './merge/useMergeQueue';
+import {
+  clampMergeInsertAt,
+  createMergedPdfName,
+  defaultMergeInsertPosition,
+} from './merge/mergeQueueHelpers';
+import type { MergeMode } from './merge/mergeTypes';
 
 function isEditableKeyboardTarget(target: EventTarget | null): boolean {
   if (!(target instanceof HTMLElement)) return false;
@@ -80,13 +88,23 @@ const App: React.FC = () => {
 
   const [previewPageId, setPreviewPageId] = useState<string | null>(null);
 
-  const [pendingFile, setPendingFile] = useState<File | null>(null);
-  const [showMergeOptions, setShowMergeOptions] = useState(false);
-  const [mergeMode, setMergeMode] = useState<
-    'overwrite' | 'append' | 'insertAt'
-  >('append');
+  const [mergeQueueOpen, setMergeQueueOpen] = useState(false);
+  const [mergeMode, setMergeMode] = useState<MergeMode>('append');
   const [mergeInsertAt, setMergeInsertAt] = useState<string>('');
 
+  const {
+    items: mergeQueueItems,
+    addFiles: addFilesToMergeQueue,
+    removeItem: removeMergeQueueItem,
+    moveItemUp: moveMergeQueueItemUp,
+    moveItemDown: moveMergeQueueItemDown,
+    clearQueue: clearMergeQueue,
+    readyPdfs: readyMergePdfs,
+    canMerge: canMergeQueue,
+    hasErrors: mergeQueueHasErrors,
+    isLoading: mergeQueueIsLoading,
+  } = useMergeQueue();
+
   const {
     splitDownloads,
     splitZipDownload,
@@ -422,89 +440,111 @@ const App: React.FC = () => {
     }
   };
 
-  const handleFileLoaded = (file: File) => {
-    if (!pdf || pages.length === 0) {
-      void loadFileAsNew(file);
-    } else {
-      setPendingFile(file);
-      setShowMergeOptions(true);
-      setMergeMode('append');
-      setMergeInsertAt(String(pages.length + 1));
+  const queueFilesForMerge = (files: File[]) => {
+    if (files.length === 0) return;
+
+    const queueWasEmpty = mergeQueueItems.length === 0;
+
+    addFilesToMergeQueue(files);
+    setMergeQueueOpen(true);
+
+    if (queueWasEmpty) {
+      if (!pdf || pages.length === 0) {
+        setMergeMode('overwrite');
+        setMergeInsertAt('1');
+      } else {
+        setMergeMode('append');
+        setMergeInsertAt(defaultMergeInsertPosition(pages.length));
+      }
     }
   };
 
+  const handleFilesLoaded = (files: File[]) => {
+    const pdfFiles = files.filter(
+      (file) =>
+        file.type === 'application/pdf' ||
+        file.name.toLowerCase().endsWith('.pdf')
+    );
+
+    if (pdfFiles.length === 0) return;
+
+    if (!pdf && pages.length === 0 && pdfFiles.length === 1) {
+      void loadFileAsNew(pdfFiles[0]);
+      return;
+    }
+
+    queueFilesForMerge(pdfFiles);
+  };
+
   const handleMergeCancel = () => {
-    setPendingFile(null);
-    setShowMergeOptions(false);
+    clearMergeQueue();
+    setMergeQueueOpen(false);
   };
 
   const handleMergeConfirm = async () => {
-    if (!pendingFile) return;
-
-    if (!pdf || mergeMode === 'overwrite') {
-      await loadFileAsNew(pendingFile);
-      setPendingFile(null);
-      setShowMergeOptions(false);
-      return;
-    }
+    if (!canMergeQueue || readyMergePdfs.length === 0) return;
 
     setError(null);
     setIsBusy(true);
 
     try {
-      // 1) Materialize the current in-memory workspace (page refs + rotations)
-      const currentBlob = await exportPages(pdf, pages);
-      const currentArrayBuffer = await currentBlob.arrayBuffer();
-      const currentDoc = await PDFDocument.load(currentArrayBuffer);
-      const currentPdf: PdfFile = {
-        id: pdf.id,
-        name: pdf.name,
-        doc: currentDoc,
-        arrayBuffer: currentArrayBuffer,
-        pageCount: pages.length,
-      };
+      let basePdf: PdfFile | null = null;
+      let insertAt = 0;
 
-      // 2) Load the new PDF
-      const newPdf = await loadPdfFromFile(pendingFile);
+      if (pdf && pages.length > 0 && mergeMode !== 'overwrite') {
+        const currentBlob = await exportPages(pdf, pages);
+        const currentArrayBuffer = await currentBlob.arrayBuffer();
+        const currentDoc = await PDFDocument.load(currentArrayBuffer);
 
-      // 3) Determine insert position (0-based)
-      let insertAt = pages.length; // default: append at end
-      if (mergeMode === 'insertAt') {
-        const parsed = parseInt(mergeInsertAt, 10);
-        if (Number.isFinite(parsed)) {
-          insertAt = Math.min(Math.max(parsed - 1, 0), pages.length);
-        }
-      } else if (mergeMode === 'append') {
-        insertAt = pages.length;
+        basePdf = {
+          id: pdf.id,
+          name: pdf.name,
+          doc: currentDoc,
+          arrayBuffer: currentArrayBuffer,
+          pageCount: pages.length,
+        };
+
+        insertAt =
+          mergeMode === 'insertAt'
+            ? clampMergeInsertAt(mergeInsertAt, pages.length)
+            : pages.length;
       }
 
-      // 4) Merge
-      const mergedPdf = await mergePdfFiles(currentPdf, newPdf, insertAt);
-      const mergedPages = createInitialPageRefs(mergedPdf.pageCount);
+      const mergedPdf = await mergePdfFilesAtPosition({
+        basePdf,
+        incomingPdfs: readyMergePdfs,
+        insertAt,
+        name: createMergedPdfName(
+          pdf?.name ?? null,
+          readyMergePdfs.map((item) => item.name),
+          mergeMode
+        ),
+      });
 
-      // 5) Reset state to the merged document
       setPdf(mergedPdf);
       replaceWorkspaceState({
-        pages: mergedPages,
+        pages: createInitialPageRefs(mergedPdf.pageCount),
         selectedPageIds: [],
         lastSelectedVisualIndex: null,
         history: [],
         redoHistory: [],
         dirty: true,
-        message: null,
+        message: `Merged ${readyMergePdfs.length} queued ${
+          readyMergePdfs.length === 1 ? 'PDF' : 'PDFs'
+        } into a new unsaved workspace.`,
       });
       clearGeneratedOutputs();
       clearThumbnailCache();
       setPreviewPageId(null);
       setWorkspaceName(defaultWorkspaceNameFromPdfName(mergedPdf.name));
       setActiveWorkspaceId(null);
+      clearMergeQueue();
+      setMergeQueueOpen(false);
     } catch (e) {
       console.error(e);
-      setError('Failed to merge PDF (see console).');
+      setError('Failed to merge PDF queue (see console).');
     } finally {
       setIsBusy(false);
-      setPendingFile(null);
-      setShowMergeOptions(false);
     }
   };
 
@@ -1100,7 +1140,29 @@ const App: React.FC = () => {
 
   return (
     <Layout onOpenHelp={() => setHelpOpen(true)}>
-      <FileLoader pdf={pdf} onFileLoaded={handleFileLoaded} />
+      <FileLoader pdf={pdf} onFilesLoaded={handleFilesLoaded} />
+
+      {mergeQueueOpen && mergeQueueItems.length > 0 && (
+        <MergeQueuePanel
+          items={mergeQueueItems}
+          hasCurrentPdf={hasPdf && pages.length > 0}
+          currentPdfName={pdf?.name ?? null}
+          currentPageCount={pages.length}
+          mergeMode={mergeMode}
+          mergeInsertAt={mergeInsertAt}
+          isBusy={isBusy}
+          canMerge={canMergeQueue}
+          isLoading={mergeQueueIsLoading}
+          hasErrors={mergeQueueHasErrors}
+          onMergeModeChange={setMergeMode}
+          onMergeInsertAtChange={setMergeInsertAt}
+          onMoveUp={moveMergeQueueItemUp}
+          onMoveDown={moveMergeQueueItemDown}
+          onRemove={removeMergeQueueItem}
+          onCancel={handleMergeCancel}
+          onConfirm={handleMergeConfirm}
+        />
+      )}
 
       <WorkspacePanel
         hasPdf={hasPdf}
@@ -1125,105 +1187,6 @@ const App: React.FC = () => {
         onRedo={handleRedo}
       />
 
-      {showMergeOptions && pendingFile && pdf && pages.length > 0 && (
-        <div
-          className="card"
-          style={{ border: '1px solid #bfdbfe', background: '#eff6ff' }}
-        >
-          <h2>Open file: merge or replace?</h2>
-          <p style={{ fontSize: '0.85rem', color: '#374151' }}>
-            You already have <strong>{pdf.name}</strong> with {pages.length}{' '}
-            pages open. What should happen with{' '}
-            <strong>{pendingFile.name}</strong>?
-          </p>
-
-          <div
-            style={{
-              display: 'flex',
-              flexDirection: 'column',
-              gap: '0.5rem',
-              marginTop: '0.5rem',
-              fontSize: '0.9rem',
-            }}
-          >
-            <label
-              style={{ display: 'flex', alignItems: 'center', gap: '0.4rem' }}
-            >
-              <input
-                type="radio"
-                name="mergeMode"
-                value="overwrite"
-                checked={mergeMode === 'overwrite'}
-                onChange={() => setMergeMode('overwrite')}
-              />
-              <span>Replace current document</span>
-            </label>
-
-            <label
-              style={{ display: 'flex', alignItems: 'center', gap: '0.4rem' }}
-            >
-              <input
-                type="radio"
-                name="mergeMode"
-                value="append"
-                checked={mergeMode === 'append'}
-                onChange={() => setMergeMode('append')}
-              />
-              <span>Merge and append pages at the end</span>
-            </label>
-
-            <label
-              style={{ display: 'flex', alignItems: 'center', gap: '0.4rem' }}
-            >
-              <input
-                type="radio"
-                name="mergeMode"
-                value="insertAt"
-                checked={mergeMode === 'insertAt'}
-                onChange={() => setMergeMode('insertAt')}
-              />
-              <span>
-                Merge and insert starting at position{' '}
-                <input
-                  type="number"
-                  min={1}
-                  max={pages.length + 1}
-                  value={mergeInsertAt}
-                  onChange={(e) => setMergeInsertAt(e.target.value)}
-                  style={{
-                    width: '4rem',
-                    padding: '0.15rem 0.3rem',
-                    fontSize: '0.85rem',
-                  }}
-                />{' '}
-                <span style={{ color: '#6b7280' }}>
-                  (1 = before first page, {pages.length + 1} = after last page)
-                </span>
-              </span>
-            </label>
-          </div>
-
-          <div className="button-row" style={{ marginTop: '0.75rem' }}>
-            <button
-              className="secondary"
-              type="button"
-              onClick={handleMergeCancel}
-              disabled={isBusy}
-            >
-              Cancel
-            </button>
-            <button
-              className="primary"
-              type="button"
-              onClick={handleMergeConfirm}
-              disabled={isBusy}
-            >
-              {isBusy ? 'Working…' : 'Continue'}
-            </button>
-          </div>
-        </div>
-      )}
-
       <ReorderPanel
         pages={pages}
         thumbnails={reorderThumbnails}

src/components/FileLoader.tsx

@@ -3,14 +3,14 @@ import type { PdfFile } from '../pdf/pdfTypes';
 
 interface FileLoaderProps {
   pdf: PdfFile | null;
-  onFileLoaded: (file: File) => void;
+  onFilesLoaded: (files: File[]) => void;
 }
 
-const FileLoader: React.FC<FileLoaderProps> = ({ pdf, onFileLoaded }) => {
+const FileLoader: React.FC<FileLoaderProps> = ({ pdf, onFilesLoaded }) => {
   const handleChange = (e: React.ChangeEvent<HTMLInputElement>) => {
-    const file = e.target.files?.[0];
-    if (file) {
-      onFileLoaded(file);
+    const files = Array.from(e.target.files ?? []);
+    if (files.length > 0) {
+      onFilesLoaded(files);
       e.target.value = '';
     }
   };
@@ -18,8 +18,16 @@ const FileLoader: React.FC<FileLoaderProps> = ({ pdf, onFileLoaded }) => {
   return (
     <div className="card">
       <h2>1. Load PDF</h2>
-      <p>Select a PDF file. Processing happens entirely in your browser.</p>
-      <input type="file" accept="application/pdf" onChange={handleChange} />
+      <p>
+        Select one PDF to open it directly, or select several PDFs to place them
+        in the merge queue. Processing happens entirely in your browser.
+      </p>
+      <input
+        type="file"
+        accept="application/pdf"
+        multiple
+        onChange={handleChange}
+      />
 
       {pdf && (
         <div style={{ marginTop: '0.75rem', fontSize: '0.9rem' }}>

src/components/HelpDialog.tsx

@@ -34,7 +34,7 @@ const shortcuts = [
 const tutorialSteps = [
   {
     title: '1. Open a PDF or load a workspace',
-    body: 'Start by selecting a local PDF file. If you saved workspaces before, you can restore one from browser storage instead.',
+    body: 'Start by selecting a local PDF file. Select several PDFs to open the merge queue, or restore a saved workspace from browser storage.',
   },
   {
     title: '2. Arrange pages visually',
@@ -50,7 +50,7 @@ const tutorialSteps = [
   },
   {
     title: '5. Split and download results',
-    body: 'Splitting creates individual one-page PDF downloads and a ZIP archive that contains all generated page files.',
+    body: 'Splitting creates individual one-page PDF downloads and a ZIP archive that contains all generated page files. For merging, review the incoming PDF queue, reorder it if needed, then replace, append, or insert the queued PDFs.',
   },
   {
     title: '6. Save your workspace or export a PDF',
@@ -102,7 +102,7 @@ const HelpDialog: React.FC<HelpDialogProps> = ({ open, onClose }) => {
             <h2 id="help-dialog-title">Help & tutorial</h2>
             <p>
               PDF Workbench is a browser-only page workspace. Use it to quickly
-              rearrange, split, merge, rotate, duplicate, and export PDFs
+              rearrange, split, queue-merge, rotate, duplicate, and export PDFs
               without uploading documents to a server.
             </p>
           </div>

src/components/MergeQueuePanel.tsx

@@ -0,0 +1,232 @@
+import React from 'react';
+import type { MergeQueueItem, MergeMode } from '../merge/mergeTypes';
+
+interface MergeQueuePanelProps {
+  items: MergeQueueItem[];
+  hasCurrentPdf: boolean;
+  currentPdfName: string | null;
+  currentPageCount: number;
+  mergeMode: MergeMode;
+  mergeInsertAt: string;
+  isBusy: boolean;
+  canMerge: boolean;
+  isLoading: boolean;
+  hasErrors: boolean;
+  onMergeModeChange: (mode: MergeMode) => void;
+  onMergeInsertAtChange: (value: string) => void;
+  onMoveUp: (itemId: string) => void;
+  onMoveDown: (itemId: string) => void;
+  onRemove: (itemId: string) => void;
+  onCancel: () => void;
+  onConfirm: () => void;
+}
+
+function formatFileSize(size: number): string {
+  if (size < 1024) return `${size} B`;
+  if (size < 1024 * 1024) return `${(size / 1024).toFixed(1)} KB`;
+  return `${(size / (1024 * 1024)).toFixed(1)} MB`;
+}
+
+const MergeQueuePanel: React.FC<MergeQueuePanelProps> = ({
+  items,
+  hasCurrentPdf,
+  currentPdfName,
+  currentPageCount,
+  mergeMode,
+  mergeInsertAt,
+  isBusy,
+  canMerge,
+  isLoading,
+  hasErrors,
+  onMergeModeChange,
+  onMergeInsertAtChange,
+  onMoveUp,
+  onMoveDown,
+  onRemove,
+  onCancel,
+  onConfirm,
+}) => {
+  const queuedPageCount = items.reduce(
+    (sum, item) => sum + (item.pageCount ?? 0),
+    0
+  );
+
+  return (
+    <div
+      className="card merge-queue-card"
+      style={{ border: '1px solid #bfdbfe', background: '#eff6ff' }}
+    >
+      <h2>Merge PDF queue</h2>
+      <p style={{ fontSize: '0.85rem', color: '#374151' }}>
+        Add several PDFs, reorder the queue, then merge them into a new unsaved
+        workspace. Processing still happens entirely in your browser.
+      </p>
+
+      {hasCurrentPdf ? (
+        <p style={{ fontSize: '0.85rem', color: '#374151' }}>
+          Current workspace:{' '}
+          <strong>{currentPdfName ?? 'Untitled document'}</strong> with{' '}
+          {currentPageCount} {currentPageCount === 1 ? 'page' : 'pages'}.
+        </p>
+      ) : (
+        <p style={{ fontSize: '0.85rem', color: '#374151' }}>
+          No current workspace is open. The queue will become a new workspace.
+        </p>
+      )}
+
+      <div className="merge-queue-list" aria-label="PDF merge queue">
+        {items.map((item, index) => (
+          <div key={item.id} className="merge-queue-item">
+            <div className="merge-queue-order">#{index + 1}</div>
+            <div className="merge-queue-details">
+              <strong>{item.name}</strong>
+              <span>
+                {formatFileSize(item.size)} ·{' '}
+                {item.status === 'ready' && item.pageCount != null
+                  ? `${item.pageCount} ${item.pageCount === 1 ? 'page' : 'pages'}`
+                  : item.status === 'loading'
+                    ? 'Loading…'
+                    : (item.error ?? 'Error')}
+              </span>
+            </div>
+            <div className="merge-queue-actions">
+              <button
+                type="button"
+                className="secondary"
+                onClick={() => onMoveUp(item.id)}
+                disabled={isBusy || index === 0}
+                title="Move up"
+              >
+                ↑
+              </button>
+              <button
+                type="button"
+                className="secondary"
+                onClick={() => onMoveDown(item.id)}
+                disabled={isBusy || index === items.length - 1}
+                title="Move down"
+              >
+                ↓
+              </button>
+              <button
+                type="button"
+                className="secondary"
+                onClick={() => onRemove(item.id)}
+                disabled={isBusy}
+                title="Remove from queue"
+              >
+                Remove
+              </button>
+            </div>
+          </div>
+        ))}
+      </div>
+
+      <p style={{ fontSize: '0.85rem', color: '#374151' }}>
+        Queue total:{' '}
+        <strong>
+          {items.length} {items.length === 1 ? 'PDF' : 'PDFs'}
+        </strong>
+        {queuedPageCount > 0 && (
+          <>
+            {' '}
+            · {queuedPageCount} {queuedPageCount === 1 ? 'page' : 'pages'}
+          </>
+        )}
+      </p>
+
+      <div className="merge-mode-group">
+        <label>
+          <input
+            type="radio"
+            name="mergeMode"
+            value="overwrite"
+            checked={mergeMode === 'overwrite'}
+            onChange={() => onMergeModeChange('overwrite')}
+            disabled={isBusy}
+          />
+          <span>
+            {hasCurrentPdf ? 'Replace current document' : 'Create from queue'}
+          </span>
+        </label>
+
+        <label>
+          <input
+            type="radio"
+            name="mergeMode"
+            value="append"
+            checked={mergeMode === 'append'}
+            onChange={() => onMergeModeChange('append')}
+            disabled={isBusy || !hasCurrentPdf}
+          />
+          <span>Append queue after current workspace</span>
+        </label>
+
+        <label>
+          <input
+            type="radio"
+            name="mergeMode"
+            value="insertAt"
+            checked={mergeMode === 'insertAt'}
+            onChange={() => onMergeModeChange('insertAt')}
+            disabled={isBusy || !hasCurrentPdf}
+          />
+          <span>
+            Insert queue starting at position{' '}
+            <input
+              type="number"
+              min={1}
+              max={currentPageCount + 1}
+              value={mergeInsertAt}
+              onChange={(e) => onMergeInsertAtChange(e.target.value)}
+              disabled={isBusy || !hasCurrentPdf || mergeMode !== 'insertAt'}
+              style={{
+                width: '4rem',
+                padding: '0.15rem 0.3rem',
+                fontSize: '0.85rem',
+              }}
+            />{' '}
+            <span style={{ color: '#6b7280' }}>
+              (1 = before first page, {currentPageCount + 1} = after last page)
+            </span>
+          </span>
+        </label>
+      </div>
+
+      {mergeMode === 'overwrite' && hasCurrentPdf && (
+        <p className="merge-warning">
+          Replace mode discards the current in-memory workspace after the merge.
+          Save it first if you want to keep the current state separately.
+        </p>
+      )}
+
+      {hasErrors && (
+        <p className="merge-warning">
+          One or more queued files could not be loaded. Remove failed items
+          before merging.
+        </p>
+      )}
+
+      <div className="button-row" style={{ marginTop: '0.75rem' }}>
+        <button
+          className="secondary"
+          type="button"
+          onClick={onCancel}
+          disabled={isBusy}
+        >
+          Cancel
+        </button>
+        <button
+          className="primary"
+          type="button"
+          onClick={onConfirm}
+          disabled={isBusy || isLoading || !canMerge || hasErrors}
+        >
+          {isBusy ? 'Working…' : isLoading ? 'Loading PDFs…' : 'Merge queue'}
+        </button>
+      </div>
+    </div>
+  );
+};
+
+export default MergeQueuePanel;

src/merge/mergeQueueHelpers.test.ts

@@ -0,0 +1,82 @@
+import { describe, expect, it } from 'vitest';
+import type { MergeQueueItem } from './mergeTypes';
+import {
+  canMergeQueue,
+  clampMergeInsertAt,
+  createMergedPdfName,
+  getReadyMergeQueuePdfs,
+  moveMergeQueueItem,
+} from './mergeQueueHelpers';
+
+function makeItem(
+  id: string,
+  status: MergeQueueItem['status']
+): MergeQueueItem {
+  return {
+    id,
+    file: new File(['x'], `${id}.pdf`, { type: 'application/pdf' }),
+    name: `${id}.pdf`,
+    size: 1,
+    pageCount: status === 'ready' ? 1 : null,
+    pdf:
+      status === 'ready'
+        ? {
+            id: `pdf-${id}`,
+            name: `${id}.pdf`,
+            // The helper tests never dereference the PDFDocument.
+            doc: {} as never,
+            pageCount: 1,
+            arrayBuffer: new ArrayBuffer(0),
+          }
+        : null,
+    status,
+  };
+}
+
+describe('merge queue helpers', () => {
+  it('moves queued items up and down without mutating the original array', () => {
+    const items = [
+      makeItem('a', 'ready'),
+      makeItem('b', 'ready'),
+      makeItem('c', 'ready'),
+    ];
+
+    expect(moveMergeQueueItem(items, 'b', 'up').map((item) => item.id)).toEqual(
+      ['b', 'a', 'c']
+    );
+    expect(
+      moveMergeQueueItem(items, 'b', 'down').map((item) => item.id)
+    ).toEqual(['a', 'c', 'b']);
+    expect(items.map((item) => item.id)).toEqual(['a', 'b', 'c']);
+  });
+
+  it('only allows merging when every queued item is ready', () => {
+    const readyItems = [makeItem('a', 'ready'), makeItem('b', 'ready')];
+    const mixedItems = [makeItem('a', 'ready'), makeItem('b', 'loading')];
+
+    expect(canMergeQueue(readyItems)).toBe(true);
+    expect(getReadyMergeQueuePdfs(readyItems)).toHaveLength(2);
+    expect(canMergeQueue(mixedItems)).toBe(false);
+    expect(canMergeQueue([])).toBe(false);
+  });
+
+  it('clamps one-based merge positions to zero-based insert slots', () => {
+    expect(clampMergeInsertAt('1', 10)).toBe(0);
+    expect(clampMergeInsertAt('5', 10)).toBe(4);
+    expect(clampMergeInsertAt('99', 10)).toBe(10);
+    expect(clampMergeInsertAt('-10', 10)).toBe(0);
+    expect(clampMergeInsertAt('not-a-number', 10)).toBe(10);
+  });
+
+  it('creates readable merged PDF filenames', () => {
+    expect(createMergedPdfName('base.pdf', ['a.pdf', 'b.pdf'], 'append')).toBe(
+      'base_plus_2_pdfs.pdf'
+    );
+    expect(createMergedPdfName('base.pdf', ['a.pdf'], 'overwrite')).toBe(
+      'a_merged.pdf'
+    );
+    expect(createMergedPdfName(null, ['a.pdf', 'b.pdf'], 'overwrite')).toBe(
+      'merged_2_pdfs.pdf'
+    );
+  });
+});

src/merge/mergeQueueHelpers.ts

@@ -0,0 +1,86 @@
+import type { PdfFile } from '../pdf/pdfTypes';
+import type { MergeMode, MergeQueueItem } from './mergeTypes';
+
+export function createMergeQueueItemId(): string {
+  return `merge_${Math.random().toString(36).slice(2)}`;
+}
+
+export function moveMergeQueueItem(
+  items: MergeQueueItem[],
+  itemId: string,
+  direction: 'up' | 'down'
+): MergeQueueItem[] {
+  const index = items.findIndex((item) => item.id === itemId);
+  if (index < 0) return items;
+
+  const targetIndex = direction === 'up' ? index - 1 : index + 1;
+  if (targetIndex < 0 || targetIndex >= items.length) return items;
+
+  const next = [...items];
+  const [item] = next.splice(index, 1);
+  next.splice(targetIndex, 0, item);
+  return next;
+}
+
+export function getReadyMergeQueuePdfs(items: MergeQueueItem[]): PdfFile[] {
+  return items
+    .filter((item) => item.status === 'ready' && item.pdf)
+    .map((item) => item.pdf as PdfFile);
+}
+
+export function canMergeQueue(items: MergeQueueItem[]): boolean {
+  return (
+    items.length > 0 &&
+    items.every((item) => item.status === 'ready' && item.pdf !== null)
+  );
+}
+
+export function hasMergeQueueErrors(items: MergeQueueItem[]): boolean {
+  return items.some((item) => item.status === 'error');
+}
+
+export function isMergeQueueLoading(items: MergeQueueItem[]): boolean {
+  return items.some((item) => item.status === 'loading');
+}
+
+export function clampMergeInsertAt(value: string, pageCount: number): number {
+  const parsed = Number.parseInt(value, 10);
+  if (!Number.isFinite(parsed)) return pageCount;
+
+  return Math.min(Math.max(parsed - 1, 0), pageCount);
+}
+
+export function defaultMergeInsertPosition(pageCount: number): string {
+  return String(pageCount + 1);
+}
+
+export function createMergedPdfName(
+  currentPdfName: string | null,
+  incomingPdfNames: string[],
+  mode: MergeMode
+): string {
+  const incomingBaseNames = incomingPdfNames.map(stripPdfExtension);
+
+  if (incomingBaseNames.length === 0) {
+    return currentPdfName ?? 'merged.pdf';
+  }
+
+  if (
+    incomingBaseNames.length === 1 &&
+    (!currentPdfName || mode === 'overwrite')
+  ) {
+    return `${incomingBaseNames[0]}_merged.pdf`;
+  }
+
+  if (currentPdfName && mode !== 'overwrite') {
+    const currentBaseName = stripPdfExtension(currentPdfName);
+    return `${currentBaseName}_plus_${incomingBaseNames.length}_pdfs.pdf`;
+  }
+
+  return `merged_${incomingBaseNames.length}_pdfs.pdf`;
+}
+
+function stripPdfExtension(filename: string): string {
+  const base = filename.replace(/\.pdf$/i, '').trim();
+  return base || 'document';
+}

src/merge/mergeTypes.ts

@@ -0,0 +1,16 @@
+import type { PdfFile } from '../pdf/pdfTypes';
+
+export type MergeMode = 'overwrite' | 'append' | 'insertAt';
+
+export type MergeQueueItemStatus = 'loading' | 'ready' | 'error';
+
+export interface MergeQueueItem {
+  id: string;
+  file: File;
+  name: string;
+  size: number;
+  pageCount: number | null;
+  pdf: PdfFile | null;
+  status: MergeQueueItemStatus;
+  error?: string;
+}

src/merge/useMergeQueue.ts

@@ -0,0 +1,102 @@
+import { useCallback, useState } from 'react';
+import { loadPdfFromFile } from '../pdf/pdfService';
+import {
+  canMergeQueue,
+  createMergeQueueItemId,
+  getReadyMergeQueuePdfs,
+  hasMergeQueueErrors,
+  isMergeQueueLoading,
+  moveMergeQueueItem,
+} from './mergeQueueHelpers';
+import type { MergeQueueItem } from './mergeTypes';
+
+export function useMergeQueue() {
+  const [items, setItems] = useState<MergeQueueItem[]>([]);
+
+  const addFiles = useCallback((files: File[]) => {
+    const pdfFiles = files.filter(
+      (file) =>
+        file.type === 'application/pdf' ||
+        file.name.toLowerCase().endsWith('.pdf')
+    );
+
+    if (pdfFiles.length === 0) return;
+
+    const queuedItems: MergeQueueItem[] = pdfFiles.map((file) => ({
+      id: createMergeQueueItemId(),
+      file,
+      name: file.name,
+      size: file.size,
+      pageCount: null,
+      pdf: null,
+      status: 'loading',
+    }));
+
+    setItems((current) => [...current, ...queuedItems]);
+
+    queuedItems.forEach((item) => {
+      void (async () => {
+        try {
+          const loadedPdf = await loadPdfFromFile(item.file);
+
+          setItems((current) =>
+            current.map((currentItem) =>
+              currentItem.id === item.id
+                ? {
+                    ...currentItem,
+                    pdf: loadedPdf,
+                    pageCount: loadedPdf.pageCount,
+                    status: 'ready',
+                    error: undefined,
+                  }
+                : currentItem
+            )
+          );
+        } catch (error) {
+          console.error(error);
+
+          setItems((current) =>
+            current.map((currentItem) =>
+              currentItem.id === item.id
+                ? {
+                    ...currentItem,
+                    status: 'error',
+                    error: 'Could not load this PDF.',
+                  }
+                : currentItem
+            )
+          );
+        }
+      })();
+    });
+  }, []);
+
+  const removeItem = useCallback((itemId: string) => {
+    setItems((current) => current.filter((item) => item.id !== itemId));
+  }, []);
+
+  const moveItemUp = useCallback((itemId: string) => {
+    setItems((current) => moveMergeQueueItem(current, itemId, 'up'));
+  }, []);
+
+  const moveItemDown = useCallback((itemId: string) => {
+    setItems((current) => moveMergeQueueItem(current, itemId, 'down'));
+  }, []);
+
+  const clearQueue = useCallback(() => {
+    setItems([]);
+  }, []);
+
+  return {
+    items,
+    addFiles,
+    removeItem,
+    moveItemUp,
+    moveItemDown,
+    clearQueue,
+    readyPdfs: getReadyMergeQueuePdfs(items),
+    canMerge: canMergeQueue(items),
+    hasErrors: hasMergeQueueErrors(items),
+    isLoading: isMergeQueueLoading(items),
+  };
+}

src/pdf/pdfMergeService.test.ts

@@ -0,0 +1,57 @@
+import { PDFDocument } from 'pdf-lib';
+import { describe, expect, it } from 'vitest';
+import type { PdfFile } from './pdfTypes';
+import { mergePdfFilesAtPosition } from './pdfService';
+
+async function makePdf(name: string, pageCount: number): Promise<PdfFile> {
+  const doc = await PDFDocument.create();
+  for (let i = 0; i < pageCount; i += 1) {
+    doc.addPage([100, 100]);
+  }
+
+  const bytes = await doc.save();
+  const arrayBuffer = new ArrayBuffer(bytes.byteLength);
+  new Uint8Array(arrayBuffer).set(bytes);
+
+  return {
+    id: name,
+    name,
+    doc,
+    pageCount,
+    arrayBuffer,
+  };
+}
+
+describe('mergePdfFilesAtPosition', () => {
+  it('merges a queue without a current base PDF', async () => {
+    const first = await makePdf('first.pdf', 1);
+    const second = await makePdf('second.pdf', 2);
+
+    const merged = await mergePdfFilesAtPosition({
+      basePdf: null,
+      incomingPdfs: [first, second],
+      insertAt: 0,
+      name: 'merged.pdf',
+    });
+
+    expect(merged.name).toBe('merged.pdf');
+    expect(merged.pageCount).toBe(3);
+    expect(merged.doc.getPageCount()).toBe(3);
+  });
+
+  it('inserts queued PDFs into a current base PDF at the requested slot', async () => {
+    const base = await makePdf('base.pdf', 3);
+    const incoming = await makePdf('incoming.pdf', 2);
+
+    const merged = await mergePdfFilesAtPosition({
+      basePdf: base,
+      incomingPdfs: [incoming],
+      insertAt: 1,
+      name: 'base_plus_incoming.pdf',
+    });
+
+    expect(merged.name).toBe('base_plus_incoming.pdf');
+    expect(merged.pageCount).toBe(5);
+    expect(merged.doc.getPageCount()).toBe(5);
+  });
+});

src/pdf/pdfService.ts

@@ -77,6 +77,76 @@ export async function mergePdfFiles(
   };
 }
 
+interface MergePdfFilesAtPositionOptions {
+  basePdf: PdfFile | null;
+  incomingPdfs: PdfFile[];
+  insertAt: number;
+  name: string;
+}
+
+export async function mergePdfFilesAtPosition({
+  basePdf,
+  incomingPdfs,
+  insertAt,
+  name,
+}: MergePdfFilesAtPositionOptions): Promise<PdfFile> {
+  if (!basePdf && incomingPdfs.length === 0) {
+    throw new Error('At least one PDF is required for merging');
+  }
+
+  const mergedDoc = await PDFDocument.create();
+
+  const addAllPages = async (sourcePdf: PdfFile) => {
+    const sourceDoc =
+      sourcePdf.doc ?? (await PDFDocument.load(sourcePdf.arrayBuffer));
+    const pageCount = sourceDoc.getPageCount();
+    const pages = await mergedDoc.copyPages(
+      sourceDoc,
+      Array.from({ length: pageCount }, (_, i) => i)
+    );
+    pages.forEach((page) => mergedDoc.addPage(page));
+  };
+
+  if (!basePdf) {
+    for (const incomingPdf of incomingPdfs) {
+      await addAllPages(incomingPdf);
+    }
+  } else {
+    const baseDoc =
+      basePdf.doc ?? (await PDFDocument.load(basePdf.arrayBuffer));
+    const basePageCount = baseDoc.getPageCount();
+    const clampedInsertAt = Math.min(Math.max(insertAt, 0), basePageCount);
+
+    const basePages = await mergedDoc.copyPages(
+      baseDoc,
+      Array.from({ length: basePageCount }, (_, i) => i)
+    );
+
+    for (let i = 0; i < clampedInsertAt; i += 1) {
+      mergedDoc.addPage(basePages[i]);
+    }
+
+    for (const incomingPdf of incomingPdfs) {
+      await addAllPages(incomingPdf);
+    }
+
+    for (let i = clampedInsertAt; i < basePages.length; i += 1) {
+      mergedDoc.addPage(basePages[i]);
+    }
+  }
+
+  const bytes = await mergedDoc.save();
+  const buffer = pdfBytesToArrayBuffer(bytes);
+
+  return {
+    id: createId(),
+    name,
+    arrayBuffer: buffer,
+    pageCount: mergedDoc.getPageCount(),
+    doc: mergedDoc,
+  };
+}
+
 export async function splitIntoSinglePages(
   pdf: PdfFile
 ): Promise<SplitResult[]> {

src/styles.css

@@ -398,3 +398,94 @@ button.secondary {
     align-items: flex-start;
   }
 }
+
+.merge-queue-list {
+  display: flex;
+  flex-direction: column;
+  gap: 0.5rem;
+  margin: 0.75rem 0;
+}
+
+.merge-queue-item {
+  display: grid;
+  grid-template-columns: auto 1fr auto;
+  align-items: center;
+  gap: 0.75rem;
+  border: 1px solid #bfdbfe;
+  border-radius: 0.75rem;
+  background: rgba(255, 255, 255, 0.8);
+  padding: 0.6rem;
+}
+
+.merge-queue-order {
+  border-radius: 999px;
+  background: #dbeafe;
+  color: #1e3a8a;
+  font-size: 0.8rem;
+  font-weight: 700;
+  padding: 0.2rem 0.5rem;
+}
+
+.merge-queue-details {
+  display: flex;
+  flex-direction: column;
+  gap: 0.1rem;
+  min-width: 0;
+  font-size: 0.9rem;
+}
+
+.merge-queue-details strong {
+  overflow-wrap: anywhere;
+}
+
+.merge-queue-details span {
+  color: #4b5563;
+  font-size: 0.8rem;
+}
+
+.merge-queue-actions {
+  display: flex;
+  flex-wrap: wrap;
+  justify-content: flex-end;
+  gap: 0.35rem;
+}
+
+.merge-queue-actions button.secondary {
+  padding: 0.3rem 0.55rem;
+  font-size: 0.8rem;
+}
+
+.merge-mode-group {
+  display: flex;
+  flex-direction: column;
+  gap: 0.5rem;
+  margin-top: 0.75rem;
+  font-size: 0.9rem;
+}
+
+.merge-mode-group label {
+  display: flex;
+  align-items: center;
+  gap: 0.4rem;
+}
+
+.merge-warning {
+  margin: 0.75rem 0 0;
+  border: 1px solid #fed7aa;
+  border-radius: 0.5rem;
+  background: #fff7ed;
+  color: #9a3412;
+  padding: 0.55rem 0.65rem;
+  font-size: 0.85rem;
+}
+
+@media (max-width: 700px) {
+  .merge-queue-item {
+    grid-template-columns: 1fr;
+    align-items: stretch;
+  }
+
+  .merge-queue-actions {
+    justify-content: flex-start;
+  }
+}

src/version.ts

@@ -1 +1 @@
-export const APP_VERSION = '0.3.1';
+export const APP_VERSION = '0.3.2';