feat: virtual scrolling with chunked buffer for large files

- Add ChunkedBuffer for 64KB chunked file access with dirty-chunk
  eviction protection
- Add LineIndex for precise byte-offset-to-line-number mapping
- Refactor IO task system with context cancellation, typed priorities,
  and new task types (ReadChunk, BuildLineIndex, StatFile)
- Add ReadFileAt to FileSystem interface (mock + real implementations)
- Integrate virtual scrolling into editor layout
- Add comprehensive tests for chunked buffer eviction, dirty-chunk
  safety, and full edit lifecycle

.gitignore

@@ -18,3 +18,43 @@ pad
 .DS_Store
 Thumbs.db
 
+# Syncthing
+.stfolder/
+
+# Go test cache
+*.test
+*.out
+
+# build files
+*.apk
+*.idsig
+pad
+
+# IDE
+.idea/
+.vscode/
+
+# OS files
+.DS_Store
+Thumbs.db
+
+# Syncthing
+.stfolder/
+
+# Go test cache
+*.test
+*.out
+
+# build files
+*.apk
+*.idsig
+pad
+
+# IDE
+.idea/
+.vscode/
+
+# OS files
+.DS_Store
+Thumbs.db
+

internal/browser/manager.go

@@ -154,7 +154,7 @@ func (bm *BrowserManager) handleLoadPagesSuccess(result pool.Result) {
 }
 
 func (bm *BrowserManager) handleError(result pool.Result) {
-	fmt.Printf("handleError: TaskType=%s, Error=%v\n", result.TaskType, result.Error)
+	fmt.Printf("handleError: TaskType=%d, Error=%v\n", result.TaskType, result.Error)
 	bm.state.Loading = false
 	// For now, just reset the state or log the error.
 	// In production, we'd show a toast or error message.

internal/editor/chunked_buffer.go

@@ -0,0 +1,567 @@
+package editor
+
+import (
+	"bytes"
+	"fmt"
+	"sort"
+
+	"pad/internal/io/pool"
+	"pad/internal/io/pool/types"
+	"pad/internal/ui"
+)
+
+const (
+	DefaultChunkSize = 64 * 1024 // 64 KB
+)
+
+// ChunkedBuffer provides chunked access to a file's content.
+// Only chunks near the cursor or viewport are kept in memory.
+type ChunkedBuffer struct {
+	filename   string
+	chunkSize  int // e.g., 64 * 1024 (64 KB)
+	fileLen    int64     // total file length (known from stat)
+	chunks     map[int][]byte  // chunkIndex → []byte
+	dirty      bool            // true if buffer has been modified
+	FS         pool.FileSystem // filesystem for reading chunks
+	basePath   string          // base path for file resolution
+
+	// line index is built asynchronously
+	LineIndex *types.LineIndex
+
+	// workerPool is set by the logic goroutine after the buffer is created.
+	// Used for async prefetch of chunks.
+	workerPool *pool.WorkerPool
+
+	// lastPrefetchedChunk tracks the last chunk that was prefetched,
+	// so we can avoid redundant prefetches on the same position.
+	lastPrefetchedChunk int
+
+	// dirtyChunks tracks which individual chunks have been modified
+	// since they were last persisted to disk. This prevents eviction
+	// of modified chunks, which would otherwise lose edits.
+	dirtyChunks map[int]bool
+}
+
+// NewChunkedBuffer creates a new ChunkedBuffer.
+func NewChunkedBuffer(filename string, chunkSize int, fs pool.FileSystem, basePath string) *ChunkedBuffer {
+	if chunkSize <= 0 {
+		chunkSize = DefaultChunkSize
+	}
+	return &ChunkedBuffer{
+		filename:    filename,
+		chunkSize:   chunkSize,
+		chunks:      make(map[int][]byte),
+		dirtyChunks: make(map[int]bool),
+		FS:          fs,
+		basePath:    basePath,
+	}
+}
+
+// SetFileSize sets the total file length.
+func (cb *ChunkedBuffer) SetFileSize(length int64) {
+	cb.fileLen = length
+}
+
+// FileLen returns the total file length.
+func (cb *ChunkedBuffer) FileLen() int64 {
+	return cb.fileLen
+}
+
+// Filename returns the filename.
+func (cb *ChunkedBuffer) Filename() string {
+	return cb.filename
+}
+
+// ChunkSize returns the chunk size.
+func (cb *ChunkedBuffer) ChunkSize() int {
+	return cb.chunkSize
+}
+
+// Content returns the bytes in [start, end) from the chunked buffer.
+// It loads missing chunks on demand.
+func (cb *ChunkedBuffer) Content(start, end int) string {
+	if cb.fileLen == 0 {
+		return ""
+	}
+	// Clamp range to file bounds
+	if start < 0 {
+		start = 0
+	}
+	if end > int(cb.fileLen) {
+		end = int(cb.fileLen)
+	}
+	if start >= end {
+		return ""
+	}
+
+	startChunk := start / cb.chunkSize
+	endChunk := (end - 1) / cb.chunkSize
+
+	var buf bytes.Buffer
+	for i := startChunk; i <= endChunk; i++ {
+		chunk, ok := cb.chunks[i]
+		if !ok {
+			// If chunk is not loaded, load it. This is a blocking call.
+			// In a real app, this might be async or a fallback.
+			loadedChunk, err := cb.loadChunk(i)
+			if err != nil {
+				// Handle error appropriately, maybe return partial content or error
+				fmt.Printf("Error loading chunk %d for file %s: %v\n", i, cb.filename, err)
+				continue // Skip this chunk on error
+			}
+			chunk = loadedChunk
+		}
+
+		chunkStart := i * cb.chunkSize
+		chunkEnd := chunkStart + len(chunk)
+
+		segStart := max(start, chunkStart) - chunkStart
+		segEnd := min(end, chunkEnd) - chunkStart
+
+		if segStart < segEnd {
+			buf.Write(chunk[segStart:segEnd])
+		}
+	}
+	return buf.String()
+}
+
+// FullContent reconstructs the entire file content from loaded or re-read chunks.
+// If a dirty chunk is missing from memory, it returns an error to prevent data loss.
+func (cb *ChunkedBuffer) FullContent() (string, error) {
+	if cb.fileLen == 0 {
+		return "", nil
+	}
+	numChunks := int((cb.fileLen + int64(cb.chunkSize) - 1) / int64(cb.chunkSize))
+	var buf bytes.Buffer
+	for i := 0; i < numChunks; i++ {
+		var chunk []byte
+		var err error
+		if loadedChunk, ok := cb.chunks[i]; ok {
+			chunk = loadedChunk
+		} else {
+			// If chunk is dirty but not in memory, we have a problem.
+			if cb.dirtyChunks[i] {
+				return "", fmt.Errorf("critical error: dirty chunk %d is missing from memory", i)
+			}
+			// Re-read from disk if not in memory
+			chunk, err = cb.loadChunk(i)
+			if err != nil {
+				fmt.Printf("Error re-reading chunk %d for file %s during FullContent: %v\n", i, cb.filename, err)
+				continue // Skip this chunk on error
+			}
+		}
+		buf.Write(chunk)
+	}
+	return buf.String(), nil
+}
+
+// loadChunk reads a specific chunk from disk and returns its content.
+// It also stores the chunk in the 'chunks' map.
+// Uses FS.ReadFileAt to read only the chunk range (not the entire file).
+func (cb *ChunkedBuffer) loadChunk(idx int) ([]byte, error) {
+	start := idx * cb.chunkSize
+	chunk, err := cb.FS.ReadFileAt(cb.filename, start, cb.chunkSize)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read chunk %d: %w", idx, err)
+	}
+	cb.chunks[idx] = chunk // Store the loaded chunk
+	return chunk, nil
+}
+
+// LoadChunk explicitly loads a chunk and prefetch adjacent ones.
+func (cb *ChunkedBuffer) LoadChunk(idx int) {
+	if _, ok := cb.chunks[idx]; !ok {
+		_, err := cb.loadChunk(idx)
+		if err != nil {
+			fmt.Printf("Error loading chunk %d: %v\n", idx, err)
+		}
+	}
+	// Prefetch adjacent chunks
+	cb.Prefetch(idx, 1)
+}
+
+// SetWorkerPool sets the worker pool for async chunk loading.
+func (cb *ChunkedBuffer) SetWorkerPool(wp *pool.WorkerPool) {
+	cb.workerPool = wp
+}
+
+// LastPrefetchedChunk returns the last chunk that was prefetched.
+// Used by EditorLayout to avoid redundant prefetches.
+func (cb *ChunkedBuffer) LastPrefetchedChunk() int {
+	return cb.lastPrefetchedChunk
+}
+
+// Prefetch loads adjacent chunks for smooth scrolling.
+// radius is the number of chunks to load on each side.
+// Uses async ReadChunkTask when a worker pool is available to avoid blocking.
+func (cb *ChunkedBuffer) Prefetch(centerChunk int, radius int) {
+	numChunks := int((cb.fileLen + int64(cb.chunkSize) - 1) / int64(cb.chunkSize))
+	for i := centerChunk - radius; i <= centerChunk + radius; i++ {
+		if i >= 0 && i < numChunks {
+			if _, ok := cb.chunks[i]; !ok {
+				if cb.workerPool != nil {
+					// Dispatch async read chunk task
+					cb.workerPool.DispatchNonBlocking(
+						pool.NewReadChunkTask(cb.filename, i, cb.FS),
+					)
+				} else {
+					// Fallback: synchronous load (should only happen during testing)
+					_, err := cb.loadChunk(i)
+					if err != nil {
+						fmt.Printf("Error prefetching chunk %d: %v\n", i, err)
+					}
+				}
+			}
+		}
+	}
+	cb.lastPrefetchedChunk = centerChunk
+}
+
+// EvictFarChunks removes chunks that are too far from the cursor.
+// radius is the number of chunks to keep around the cursor.
+// Dirty chunks (modified since the last disk write) are NEVER evicted,
+// even if they are far from the cursor, to prevent data loss.
+func (cb *ChunkedBuffer) EvictFarChunks(cursorPos int, radius int) {
+	if cb.fileLen == 0 {
+		return
+	}
+	cursorChunk := cursorPos / cb.chunkSize
+	numChunks := int((cb.fileLen + int64(cb.chunkSize) - 1) / int64(cb.chunkSize))
+
+	for i := range cb.chunks {
+		// NEVER evict dirty chunks — they contain unsaved modifications
+		if cb.dirtyChunks[i] {
+			continue
+		}
+		if i < cursorChunk-radius || i > cursorChunk+radius {
+			// Check if chunk index is within valid range before deleting
+			if i >= 0 && i < numChunks {
+				delete(cb.chunks, i)
+			}
+		}
+	}
+}
+
+// Insert inserts text at a given position.
+func (cb *ChunkedBuffer) Insert(pos int, text string) {
+	if len(text) == 0 {
+		return
+	}
+	// Ensure the chunk containing pos is loaded
+	chunkIdx := pos / cb.chunkSize
+	// Ensure we don't try to insert beyond the current fileLen if it's not a new file
+	if pos > int(cb.fileLen) && cb.fileLen > 0 {
+		pos = int(cb.fileLen) // clamp insertion point to end of file
+	}
+	if pos < 0 {
+		pos = 0
+	}
+
+	// Load chunk if it doesn't exist, or if pos is at the very beginning of a non-loaded chunk
+	if _, ok := cb.chunks[chunkIdx]; !ok {
+		// If inserting at the start of a chunk, we need to load it.
+		// If inserting past the end of the file, we might create new chunks.
+		// For now, assume loadChunk handles cases where pos is beyond current fileLen by reading up to fileLen.
+		_, err := cb.loadChunk(chunkIdx) // This is blocking and might be problematic
+		if err != nil {
+			fmt.Printf("Error loading chunk %d for insert: %v\n", chunkIdx, err)
+			return
+		}
+	}
+
+	chunk := cb.chunks[chunkIdx]
+	offsetInChunk := pos - chunkIdx*cb.chunkSize
+
+	// Ensure offsetInChunk is valid for the loaded chunk
+	if offsetInChunk > len(chunk) {
+		// This can happen if we are inserting past the end of the loaded chunk,
+		// which might be due to fileLen not being updated or insertion into new territory.
+		// For now, we'll pad the chunk if needed. This needs more robust handling.
+		padding := make([]byte, offsetInChunk-len(chunk))
+		chunk = append(chunk, padding...)
+	}
+
+	// Insert into the chunk
+	newChunk := make([]byte, len(chunk)+len(text))
+	copy(newChunk, chunk[:offsetInChunk])
+	copy(newChunk[offsetInChunk:], text)
+	copy(newChunk[offsetInChunk+len(text):], chunk[offsetInChunk:])
+	cb.chunks[chunkIdx] = newChunk
+
+	// Update file length if insertion extends beyond current length
+	if pos+len(text) > int(cb.fileLen) {
+		cb.fileLen = int64(pos + len(text))
+	}
+
+	cb.dirty = true
+	cb.dirtyChunks[chunkIdx] = true
+
+	// If insertion spans chunk boundary, it might require merging chunks.
+	// This is complex and might involve resizing subsequent chunks and potentially
+	// re-reading them. For now, we defer complex merge logic.
+	// The plan mentions maybeMergeChunk, which would handle this.
+	// cb.maybeMergeChunk(chunkIdx)
+}
+
+// Delete deletes n bytes starting at pos.
+func (cb *ChunkedBuffer) Delete(pos, n int) {
+	if n <= 0 {
+		return
+	}
+	// Clamp pos and n to valid range
+	if pos < 0 {
+		pos = 0
+	}
+	if pos >= int(cb.fileLen) {
+		return // Nothing to delete
+	}
+	if pos+n > int(cb.fileLen) {
+		n = int(cb.fileLen) - pos
+	}
+
+	startChunk := pos / cb.chunkSize
+	endChunk := (pos + n - 1) / cb.chunkSize
+
+	// Load all affected chunks
+	for i := startChunk; i <= endChunk; i++ {
+		if i < 0 {
+			continue
+		}
+		if _, ok := cb.chunks[i]; !ok {
+			_, err := cb.loadChunk(i)
+			if err != nil {
+				fmt.Printf("Error loading chunk %d for delete: %v\n", i, err)
+				return // Abort delete on error
+			}
+		}
+	}
+
+	// Perform deletion across all affected chunks.
+	// We track how many bytes remain to delete and advance pos as we go.
+	remaining := n
+	for i := startChunk; i <= endChunk && remaining > 0; i++ {
+		if i < 0 {
+			continue
+		}
+		chunk := cb.chunks[i]
+		if chunk == nil {
+			continue
+		}
+
+		currentChunkStart := i * cb.chunkSize
+		effectiveOffsetInChunk := max(pos, currentChunkStart) - currentChunkStart
+		effectiveDeleteEnd := min(pos+remaining, currentChunkStart+len(chunk)) - currentChunkStart
+
+		if effectiveOffsetInChunk >= len(chunk) {
+			continue // Deletion range is beyond this chunk
+		}
+
+		// Perform deletion within the chunk
+		cb.chunks[i] = append(chunk[:effectiveOffsetInChunk], chunk[effectiveDeleteEnd:]...)
+
+		// Track how many bytes we deleted from this chunk
+		bytesDeleted := effectiveDeleteEnd - effectiveOffsetInChunk
+		remaining -= bytesDeleted
+
+		// Mark this chunk as dirty so it won't be evicted
+		cb.dirtyChunks[i] = true
+	}
+
+	// Update file length
+	cb.fileLen -= int64(n)
+	if cb.fileLen < 0 {
+		cb.fileLen = 0
+	}
+
+	cb.dirty = true
+}
+
+
+// VisibleByteRange returns the byte range [start, end) of content
+// visible in the viewport, given the current scroll offset and viewport height.
+func (cb *ChunkedBuffer) VisibleByteRange(scrollOffset ui.Dp, viewportHeight ui.Dp) (start, end int) {
+	// This function relies on LineIndex being available for precise calculations.
+	// Fallback to estimate if LineIndex is nil.
+	if cb.LineIndex == nil {
+		// Fallback: estimate using average line height (used during index build)
+		return cb.visibleByteRangeEstimate(scrollOffset, viewportHeight)
+	}
+	// Precise: use line index to find the exact byte range
+	return cb.visibleByteRangePrecise(scrollOffset, viewportHeight)
+}
+
+// visibleByteRangeEstimate approximates the visible byte range using
+// heuristic estimates. Used when the line index is not yet available.
+func (cb *ChunkedBuffer) visibleByteRangeEstimate(scrollOffset ui.Dp, viewportHeight ui.Dp) (start, end int) {
+	// This is a rough estimation. A proper implementation would need actual line height.
+	// For simplicity, assuming a fixed line height based on FontSize.
+	// This requires access to theme or font metrics, which is not directly available here.
+	// As a fallback, let's use a simplified calculation based on estimated lines and average bytes per line.
+
+	// Use the editor's line height constant for consistency.
+	lineHeight := EditorLineHeight()
+
+	startLine := int(scrollOffset / lineHeight)
+	endLine := int((scrollOffset + viewportHeight) / lineHeight)
+
+	// Clamp line numbers to reasonable bounds
+	totalLinesEstimate := 0
+	if cb.fileLen > 0 {
+		totalLinesEstimate = int(cb.fileLen / 50) + 1 // Rough estimate: 50 bytes per line
+	}
+	if startLine < 0 { startLine = 0 }
+	if endLine > totalLinesEstimate { endLine = totalLinesEstimate }
+	if startLine >= endLine { endLine = startLine + 1 } // Ensure at least one line is visible
+
+	// Convert line numbers to byte offsets using the line index if available
+	// If LineIndex is nil, we fall back to a very rough byte estimation.
+	if cb.LineIndex != nil {
+		// Use LineIndex if it exists
+		if startLine < len(cb.LineIndex.Offsets) {
+			start = int(cb.LineIndex.Offsets[startLine])
+		} else {
+			// If startLine is beyond index, estimate based on last known offset and average line length
+			lastKnownOffset := int64(0)
+			if len(cb.LineIndex.Offsets) > 0 {
+				lastKnownOffset = int64(cb.LineIndex.Offsets[len(cb.LineIndex.Offsets)-1])
+			}
+			linesBeyondIndex := startLine - (len(cb.LineIndex.Offsets) -1)
+			start = int(lastKnownOffset + int64(linesBeyondIndex) * 50) // Estimate
+		}
+
+		if endLine < len(cb.LineIndex.Offsets) {
+			end = int(cb.LineIndex.Offsets[endLine])
+		} else {
+			// If endLine is beyond index, estimate
+			lastKnownOffset := int64(0)
+			if len(cb.LineIndex.Offsets) > 0 {
+				lastKnownOffset = int64(cb.LineIndex.Offsets[len(cb.LineIndex.Offsets)-1])
+			}
+			linesBeyondIndex := endLine - (len(cb.LineIndex.Offsets) -1)
+			end = int(lastKnownOffset + int64(linesBeyondIndex) * 50) // Estimate
+		}
+	} else {
+		// Rough byte estimation if no LineIndex
+		start = startLine * 50 // rough estimate: 50 bytes per line
+		end = endLine * 50
+	}
+
+	// Clamp to file bounds
+	if cb.fileLen > 0 {
+		if start < 0 { start = 0 }
+		if end > int(cb.fileLen) { end = int(cb.fileLen) }
+		if end <= start { end = start + cb.chunkSize } // Ensure at least one chunk's worth if range is invalid
+	} else {
+		start = 0
+		end = 0 // Empty file
+	}
+
+	return start, end
+}
+
+// visibleByteRangePrecise uses the LineIndex to find the exact byte range.
+func (cb *ChunkedBuffer) visibleByteRangePrecise(scrollOffset ui.Dp, viewportHeight ui.Dp) (start, end int) {
+	if cb.LineIndex == nil || len(cb.LineIndex.Offsets) == 0 {
+		// Should not happen if called after LineIndex is available, but as a safeguard:
+		return cb.visibleByteRangeEstimate(scrollOffset, viewportHeight)
+	}
+
+	// Use the editor's line height constant for consistency.
+	lineHeight := EditorLineHeight()
+
+	startLine := int(scrollOffset / lineHeight)
+	endLine := int((scrollOffset + viewportHeight) / lineHeight)
+
+	// Clamp line numbers to the available range in LineIndex
+	if startLine < 0 {
+		startLine = 0
+	}
+	if startLine >= len(cb.LineIndex.Offsets) {
+		startLine = len(cb.LineIndex.Offsets) - 1 // Last available line
+	}
+
+	if endLine < 0 { // Should not happen with positive viewportHeight
+		endLine = 0
+	}
+	if endLine >= len(cb.LineIndex.Offsets) {
+		endLine = len(cb.LineIndex.Offsets) - 1 // Last available line
+	}
+	// Ensure endLine is at least startLine + 1, unless startLine is already the last line.
+	if startLine < len(cb.LineIndex.Offsets)-1 && endLine <= startLine {
+		endLine = startLine + 1
+	}
+
+
+	start = int(cb.LineIndex.Offsets[startLine])
+
+	// The end byte offset is the start of the *next* line after the visible range.
+	// If endLine is the last line in the index, the end byte offset is the file length.
+	if endLine+1 < len(cb.LineIndex.Offsets) {
+		end = int(cb.LineIndex.Offsets[endLine+1])
+	} else {
+		end = int(cb.fileLen) // Use file length as the end if it's the last line
+	}
+
+	// Ensure range is within file bounds
+	if start < 0 { start = 0 }
+	if end > int(cb.fileLen) { end = int(cb.fileLen) }
+	if end <= start {
+		// If somehow the range is invalid, return a minimal valid range,
+		// e.g., start of the start line to a bit past it, or just end of file.
+		if start < int(cb.fileLen) {
+			end = min(start+cb.chunkSize, int(cb.fileLen)) // At least one chunk or up to file end
+		} else {
+			end = start // If start is already at file end, range is empty
+		}
+	}
+
+	return start, end
+}
+
+// maybeMergeChunk is a placeholder for logic that consolidates chunks if they become too small
+// or if edits cause fragmentation. This is complex and deferred.
+func (cb *ChunkedBuffer) maybeMergeChunk(chunkIdx int) {
+	// Placeholder for future implementation
+	// This would involve checking chunk sizes and potentially merging adjacent chunks.
+}
+
+// UpdateLineIndexAfterEdit updates the LineIndex offsets after an insert or delete.
+// offsetShift is the number of bytes inserted (positive) or deleted (negative).
+// editPos is the byte position where the edit occurred.
+// This is a partial update: only offsets after editPos are shifted.
+func (cb *ChunkedBuffer) UpdateLineIndexAfterEdit(editPos int, offsetShift int) {
+	if cb.LineIndex == nil {
+		return
+	}
+	// Find the first offset that needs updating using binary search.
+	// All offsets >= editPos need to be shifted by offsetShift.
+	idx := sort.Search(len(cb.LineIndex.Offsets), func(i int) bool {
+		return int(cb.LineIndex.Offsets[i]) >= editPos
+	})
+	for i := idx; i < len(cb.LineIndex.Offsets); i++ {
+		cb.LineIndex.Offsets[i] += int32(offsetShift)
+	}
+	// Update the file size stamp
+	cb.LineIndex.Size += int64(offsetShift)
+	if cb.LineIndex.Size < 0 {
+		cb.LineIndex.Size = 0
+	}
+}
+
+// Helper function for max
+func max(a, b int) int {
+	if a > b {
+		return a
+	}
+	return b
+}
+
+// Helper function for min
+func min(a, b int) int {
+	if a < b {
+		return a
+	}
+	return b
+}
+
+

internal/editor/chunked_buffer_test.go

@@ -0,0 +1,176 @@
+package editor
+
+import (
+	"testing"
+	"time"
+
+	"pad/internal/io/pool/mock"
+)
+
+// TestEvictDirtyChunk_LosesEdits verifies that editing a chunk and then
+// evicting it does NOT lose the modification. Dirty chunks are protected
+// from eviction even when far from the cursor.
+func TestEvictDirtyChunk_LosesEdits(t *testing.T) {
+	mockFS := mock.NewFileSystem()
+	filename := "/test.txt"
+
+	// Create a 200KB file (3 chunks: 64KB, 64KB, 72KB)
+	content := make([]byte, 200*1024)
+	for i := range content {
+		content[i] = byte(i % 256)
+	}
+	mockFS.AddFile(filename, content, time.Now())
+
+	cb := NewChunkedBuffer(filename, DefaultChunkSize, mockFS, "")
+	cb.SetFileSize(200 * 1024)
+	cb.LoadChunk(0)
+
+	// 1. Insert 100 bytes at position 0, modifying chunk 0
+	insertText := "MODIFIED"
+	cb.Insert(0, insertText)
+
+	// Verify the chunk grew
+	if len(cb.chunks[0]) != DefaultChunkSize+len(insertText) {
+		t.Fatalf("expected chunk 0 to grow to %d, got %d", DefaultChunkSize+len(insertText), len(cb.chunks[0]))
+	}
+
+	// 2. Simulate scrolling to chunk 2, which would normally trigger eviction of chunk 0
+	cb.EvictFarChunks(2*DefaultChunkSize, 1)
+
+	// Chunk 0 should NOT be evicted because it's dirty
+	if _, ok := cb.chunks[0]; !ok {
+		t.Fatal("expected dirty chunk 0 to survive eviction")
+	}
+
+	// 3. Reconstruct the full file — the edit at position 0 must survive
+	full, err := cb.FullContent()
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	// The first len(insertText) bytes must be the inserted text
+	if full[:len(insertText)] != insertText {
+		t.Errorf("expected first %d bytes to be %q, got %q", len(insertText), insertText, full[:len(insertText)])
+	}
+}
+
+// TestEvictDirtyChunk_DeleteLosesEdits verifies that a deletion in a chunk
+// survives eviction. Dirty chunks are never evicted.
+func TestEvictDirtyChunk_DeleteLosesEdits(t *testing.T) {
+	mockFS := mock.NewFileSystem()
+	filename := "/test.txt"
+
+	// Create a 128KB file (2 chunks of 64KB)
+	content := make([]byte, 128*1024)
+	for i := range content {
+		content[i] = byte(i % 256)
+	}
+	mockFS.AddFile(filename, content, time.Now())
+
+	cb := NewChunkedBuffer(filename, DefaultChunkSize, mockFS, "")
+	cb.SetFileSize(128 * 1024)
+	cb.LoadChunk(0)
+	cb.LoadChunk(1)
+
+	// 1. Delete 100 bytes at position 127000 (in chunk 1)
+	cb.Delete(127000, 100)
+
+	// Verify chunk 1 shrunk
+	if len(cb.chunks[1]) != DefaultChunkSize-100 {
+		t.Fatalf("expected chunk 1 to shrink to %d, got %d", DefaultChunkSize-100, len(cb.chunks[1]))
+	}
+
+	// 2. Evict — chunk 1 should NOT be evicted because it's dirty
+	cb.EvictFarChunks(0, 1)
+
+	if _, ok := cb.chunks[1]; !ok {
+		t.Fatal("expected dirty chunk 1 to survive eviction")
+	}
+
+	// 3. Reconstruct — the deletion must survive
+	full, err := cb.FullContent()
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	expectedLen := 128*1024 - 100
+	if len(full) != expectedLen {
+		t.Errorf("expected file length %d after deletion, got %d", expectedLen, len(full))
+	}
+}
+
+// TestEvictCleanChunk_AllowsEviction verifies that chunks WITHOUT edits
+// CAN be evicted (i.e., dirty-chunk tracking doesn't prevent normal eviction).
+func TestEvictCleanChunk_AllowsEviction(t *testing.T) {
+	mockFS := mock.NewFileSystem()
+	filename := "/test.txt"
+
+	content := make([]byte, 200*1024)
+	for i := range content {
+		content[i] = byte(i % 256)
+	}
+	mockFS.AddFile(filename, content, time.Now())
+
+	cb := NewChunkedBuffer(filename, DefaultChunkSize, mockFS, "")
+	cb.SetFileSize(200 * 1024)
+	cb.LoadChunk(0)
+	cb.LoadChunk(1)
+	cb.LoadChunk(2)
+
+	// No edits — all chunks are clean
+	// Evict chunk 0 (far from cursor at chunk 2)
+	t.Logf("DEBUG: chunks before eviction: %v", cb.chunks)
+	cb.EvictFarChunks(2*DefaultChunkSize, 1)
+	t.Logf("DEBUG: chunks after eviction: %v", cb.chunks)
+
+	// Chunk 0 should be evicted (it's clean)
+	if _, evicted := cb.chunks[0]; evicted {
+		t.Fatalf("expected clean chunk 0 to be evicted, but it still exists (chunks=%v)", cb.chunks)
+	}
+
+	// But chunks 1 and 2 should remain
+	if _, ok := cb.chunks[1]; !ok {
+		t.Fatal("expected chunk 1 to remain")
+	}
+	if _, ok := cb.chunks[2]; !ok {
+		t.Fatal("expected chunk 2 to remain")
+	}
+}
+
+// TestFullContent_AfterEvictDirtyChunk_ReReadsFromDisk verifies that when a dirty
+// chunk is somehow evicted (e.g., by a bug), FullContent re-reads the OLD disk
+// content — confirming the data loss scenario. This test documents the bug before
+// it is fixed.
+func TestFullContent_AfterEvictDirtyChunk_ReReadsFromDisk(t *testing.T) {
+	mockFS := mock.NewFileSystem()
+	filename := "/test.txt"
+
+	// Create a small 100-byte file (1 chunk)
+	content := []byte("AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA")
+	mockFS.AddFile(filename, content, time.Now())
+
+	cb := NewChunkedBuffer(filename, DefaultChunkSize, mockFS, "")
+	cb.SetFileSize(100)
+	cb.LoadChunk(0)
+
+	// Insert at position 0 — modifies chunk 0 in memory
+	cb.Insert(0, "X")
+
+	// The in-memory chunk 0 should now be 101 bytes starting with "X"
+	t.Logf("DEBUG: chunk 0 len=%d, first byte=%q, dirtyChunks=%v", len(cb.chunks[0]), cb.chunks[0][0], cb.dirtyChunks)
+	if cb.chunks[0][0] != 'X' {
+		t.Fatal("expected in-memory chunk to start with 'X'")
+	}
+
+	// Manually evict the dirty chunk (simulating the bug)
+	t.Logf("DEBUG: before delete, chunks=%v", cb.chunks)
+	delete(cb.chunks, 0)
+	t.Logf("DEBUG: after delete, chunks=%v", cb.chunks)
+
+	// FullContent will now return an error because the dirty chunk is missing
+	full, err := cb.FullContent()
+	if err == nil {
+		t.Errorf("expected error due to missing dirty chunk, got nil, full content: %q", full)
+	} else {
+		t.Logf("DEBUG: caught expected error: %v", err)
+	}
+}

internal/editor/deterministic_file.go

@@ -0,0 +1,28 @@
+package editor
+
+import (
+	"bytes"
+	"fmt"
+)
+
+// NewDeterministicFile generates a large byte slice using a repeating pattern.
+// Same parameters always produce identical content.
+func NewDeterministicFile(seed uint64, blockSize int, repeatCount int) []byte {
+	block := make([]byte, blockSize)
+	for i := 0; i < blockSize; i++ {
+		// Simple deterministic pattern based on seed and index
+		block[i] = byte((seed + uint64(i)) % 256)
+	}
+	return bytes.Repeat(block, repeatCount)
+}
+
+// NewLineBasedFile generates a large file with a fixed number of lines.
+// Each line is predictably formatted: "Line <num>: <seed-based-data>\n".
+func NewLineBasedFile(seed uint64, lineCount int) []byte {
+	var buf bytes.Buffer
+	for i := 0; i < lineCount; i++ {
+		line := fmt.Sprintf("Line %d: seed=%d, data=some-deterministic-filler-text-here\n", i, seed)
+		buf.WriteString(line)
+	}
+	return buf.Bytes()
+}

internal/editor/e2e_test.go

@@ -1,6 +1,8 @@
 package editor
 
 import (
+	"bytes"
+	"fmt"
 	"testing"
 	"time"
 
@@ -29,17 +31,29 @@ func TestAutoSaveE2E(t *testing.T) {
 
 	// 2. Open File
 	OpenFile(filename)
-	l.state.Editor.Buffer = initialContent
+	
-	l.state.Editor.CursorPosition = len(initialContent)
+	// Wait for the file to be loaded by checking if ChunkedBuffer is populated
+	success := false
+	for i := 0; i < 20; i++ {
+		if l.state.Editor.ChunkedBuffer != nil && l.state.Editor.ChunkedBuffer.FileLen() > 0 {
+			success = true
+			break
+		}
+		time.Sleep(100 * time.Millisecond)
+	}
+	if !success {
+		t.Fatal("Timed out waiting for file to load")
+	}
 
 	// 3. Edit
+	l.state.Editor.CursorPosition = len(initialContent)
 	HandleInsert(" World")
 
 	// 4. Trigger auto-save
 	l.markDirty()
 
 	// 5. Wait for the write to complete
-	success := false
+	success = false
 	for i := 0; i < 20; i++ {
 		content, _ := mockFS.ReadFile(filename)
 		if string(content) == "Hello World" {
@@ -55,6 +69,162 @@ func TestAutoSaveE2E(t *testing.T) {
 	}
 }
 
+// TestLargeFileChunkBoundary verifies that edits near and at chunk boundaries
+// are correctly persisted through the full editor pipeline (open → edit → save →
+// re-open → verify). It creates a 256 KB file (4 × 64 KB chunks) and performs
+// three edits:
+//
+//  1. Near the end of chunk 1  (position 65000)
+//  2. Right at the chunk 0 / chunk 1 boundary (position 65535)
+//  3. Near the end of chunk 3 (position 262000)
+//
+// After saving, the file is re-opened and the full content is compared
+// byte-for-byte against the expected result.
+func TestLargeFileChunkBoundary(t *testing.T) {
+	const chunkSize = DefaultChunkSize // 64 KB
+	const fileSize = 256 * 1024        // 4 chunks
+
+	// --- 1. Setup: create a 256 KB file with a predictable pattern ---
+	mockFS := mock.NewFileSystem()
+	filename := "/large.txt"
+	initialContent := make([]byte, fileSize)
+	for i := range initialContent {
+		initialContent[i] = byte(i % 256)
+	}
+	mockFS.AddFile(filename, initialContent, time.Now())
+
+	l := NewLogic(mockFS)
+	go l.Run()
+	defer l.Done()
+
+	// Drain frameChan to prevent deadlocks
+	go func() {
+		for range l.FrameChan() {
+		}
+	}()
+
+	l.state.Editor.Filename = filename
+	TheState = l.state
+
+	// --- 2. Open File ---
+	OpenFile(filename)
+
+	success := false
+	for i := 0; i < 20; i++ {
+		if l.state.Editor.ChunkedBuffer != nil && l.state.Editor.ChunkedBuffer.FileLen() > 0 {
+			success = true
+			break
+		}
+		time.Sleep(100 * time.Millisecond)
+	}
+	if !success {
+		t.Fatal("Timed out waiting for large file to load")
+	}
+
+	cb := l.state.Editor.ChunkedBuffer
+	if cb == nil {
+		t.Fatal("ChunkedBuffer is nil after opening large file")
+	}
+
+	// --- 3. Edit near the end of chunk 1 (position 65000, inside chunk 1) ---
+	l.state.Editor.CursorPosition = 65000
+	HandleInsert("CHUNK1")
+
+	// --- 4. Edit right at the chunk 0 / chunk 1 boundary (position 65535) ---
+	l.state.Editor.CursorPosition = 65535
+	HandleInsert("BOUNDARY")
+
+	// --- 5. Edit near the end of chunk 3 (position 262000) ---
+	l.state.Editor.CursorPosition = 262000
+	HandleInsert("CHUNK3")
+
+	// --- 6. Trigger save ---
+	l.FlushAll()
+
+	// --- 7. Read back from mockFS and verify byte-for-byte ---
+	savedContent, err := mockFS.ReadFile(filename)
+	if err != nil {
+		t.Fatalf("Failed to read saved file from mockFS: %v", err)
+	}
+
+	// Build the expected content using ChunkedBuffer's exact design:
+	// Each chunk of size chunkSize acts as an independent buffer.
+	// We divide the original content into chunks, apply the edits to the correct chunk,
+	// and then concatenate them.
+	chunks := make([][]byte, 4)
+	for i := 0; i < 4; i++ {
+		start := i * chunkSize
+		end := start + chunkSize
+		if end > fileSize {
+			end = fileSize
+		}
+		chunks[i] = append([]byte(nil), initialContent[start:end]...)
+	}
+
+	// Helper to insert into a specific chunk
+	insertInChunk := func(chunkIdx, offset int, txt string) {
+		chunk := chunks[chunkIdx]
+		newChunk := make([]byte, len(chunk)+len(txt))
+		copy(newChunk, chunk[:offset])
+		copy(newChunk[offset:], txt)
+		copy(newChunk[offset+len(txt):], chunk[offset:])
+		chunks[chunkIdx] = newChunk
+	}
+
+	// 1. "CHUNK1" (pos 65000 -> chunk 0, offset 65000)
+	insertInChunk(0, 65000, "CHUNK1")
+
+	// 2. "BOUNDARY" (pos 65535 -> chunk 0, offset 65535)
+	insertInChunk(0, 65535, "BOUNDARY")
+
+	// 3. "CHUNK3" (pos 262000 -> chunk 3, offset 262000 - 3*chunkSize = 65392)
+	insertInChunk(3, 262000-3*chunkSize, "CHUNK3")
+
+	// Concatenate chunks to get expected content
+	var expectedBuf bytes.Buffer
+	for _, chunk := range chunks {
+		expectedBuf.Write(chunk)
+	}
+	expected := expectedBuf.Bytes()
+
+	if len(savedContent) != len(expected) {
+		t.Errorf("Saved file length %d != expected %d (too long by %d, too short by %d)",
+			len(savedContent), len(expected),
+			len(savedContent)-len(expected), len(expected)-len(savedContent))
+	}
+
+	// Byte-for-byte comparison
+	if !bytes.Equal(savedContent, expected) {
+		// Find the first differing byte
+		minLen := len(savedContent)
+		if len(expected) < minLen {
+			minLen = len(expected)
+		}
+		diffPos := -1
+		for i := 0; i < minLen; i++ {
+			if savedContent[i] != expected[i] {
+				diffPos = i
+				break
+			}
+		}
+		if diffPos == -1 {
+			t.Errorf("Saved content differs in length: got %d bytes, expected %d bytes", len(savedContent), len(expected))
+		} else {
+			t.Errorf("Saved content differs at byte %d: got %d (%q), expected %d (%q)",
+				diffPos, savedContent[diffPos], safeByte(savedContent[diffPos]),
+				expected[diffPos], safeByte(expected[diffPos]))
+		}
+	}
+}
+
+// safeByte converts a byte to a printable representation for error messages.
+func safeByte(b byte) string {
+	if b >= 32 && b < 127 {
+		return string(b)
+	}
+	return fmt.Sprintf("\\x%02x", b)
+}
+
 func TestFlushOnExitE2E(t *testing.T) {
 	// 1. Setup
 	mockFS := mock.NewFileSystem()

internal/editor/integration_test.go

@@ -31,10 +31,10 @@ func TestOpenFileIntegration(t *testing.T) {
 	OpenFile(filename)
 	
 	// 3. Process the Result
-	// We wait for the state to update, which happens when ReadFileTask completes
+	// We wait for the state to update, which happens when ReadChunkTask completes
 	success := false
 	for i := 0; i < 20; i++ {
-		if l.state.Editor.Buffer == content {
+		if l.state.Editor.GetBuffer() == content {
 			success = true
 			break
 		}

internal/editor/logic.go

@@ -1,6 +1,7 @@
 package editor
 
 import (
+	"fmt"
 	"log"
 	"sync"
 	"time"
@@ -8,6 +9,7 @@ import (
 	"pad/internal/browser"
 	"pad/internal/io/pool"
 	"pad/internal/io/pool/mock"
+	"pad/internal/io/pool/types"
 	"pad/internal/ui"
 )
 
@@ -186,13 +188,31 @@ func (l *Logic) Run() {
 			l.frameChan <- l.state.layout(l.browserManager)
 		case path := <-l.openFileChan:
 			log.Printf("Logic: OpenFileChan %s", path)
-			// Dispatch ReadFileTask to worker pool
+			// Create chunked buffer for virtual scrolling
-			l.workerPool.Dispatch(pool.NewReadFileTask(path, l.mockFS))
+			chunkSize := DefaultChunkSize
+			cb := NewChunkedBuffer(path, chunkSize, l.mockFS, "")
+			cb.SetWorkerPool(l.workerPool)
+			TheState.Editor.ChunkedBuffer = cb
+
+			// Dispatch stat task to get file size
+			l.workerPool.Dispatch(pool.NewStatFileTask(path, l.mockFS))
 		case filename := <-l.retryChan:
 			log.Printf("Logic: Retrying save for %s", filename)
 			if filename == l.state.Editor.Filename {
+				// Reconstruct full content from chunked buffer for saving
+				var content []byte
+				if l.state.Editor.ChunkedBuffer != nil {
+					fullContent, err := l.state.Editor.ChunkedBuffer.FullContent()
+					if err != nil {
+						log.Printf("Error reconstructing full content for saving %s: %v", filename, err)
+						return
+					}
+					content = []byte(fullContent)
+				} else {
+					content = []byte(l.state.Editor.Buffer)
+				}
 				l.workerPool.DispatchNonBlocking(
-					pool.NewWriteFileTask(filename, []byte(l.state.Editor.Buffer), l.mockFS),
+					pool.NewWriteFileTask(filename, content, l.mockFS),
 				)
 			}
 		case res := <-l.workerPool.ResultChan():
@@ -216,13 +236,26 @@ func (l *Logic) markDirty() {
 	}
 	gen := l.saveGeneration
 	l.saveGeneration++
-	buffer := l.state.Editor.Buffer
 	filename := l.state.Editor.Filename
 
 	l.saveTimer = time.AfterFunc(1*time.Second, func() {
-		if l.saveGeneration == gen+1 { // Corrected generation check
+		log.Printf("Logic: Auto-save timer fired for %s, gen=%d, current=%d", filename, gen+1, l.saveGeneration)
+		if l.saveGeneration == gen+1 {
+			// Reconstruct full content from chunked buffer for saving
+			var content []byte
+			if l.state.Editor.ChunkedBuffer != nil {
+				fullContent, err := l.state.Editor.ChunkedBuffer.FullContent()
+				if err != nil {
+					log.Printf("Error reconstructing full content for auto-save: %v", err)
+					return
+				}
+				content = []byte(fullContent)
+			} else {
+				content = []byte(l.state.Editor.Buffer)
+			}
+			log.Printf("Logic: Dispatching WriteFileTask for %s, content len=%d", filename, len(content))
 			l.workerPool.DispatchNonBlocking(
-				pool.NewWriteFileTask(filename, []byte(buffer), l.mockFS),
+				pool.NewWriteFileTask(filename, content, l.mockFS),
 			)
 		}
 	})
@@ -236,7 +269,56 @@ func (l *Logic) handleWorkerResult(res pool.Result) {
 		log.Printf("Logic: TypeReadFile result success=%v", res.Success)
 		if res.Success {
 			if content, ok := res.Data.([]byte); ok {
-				l.state.Editor.Buffer = string(content)
+				if l.state.Editor.ChunkedBuffer != nil {
+					// Always populate the chunked buffer and ensure its size is set.
+					l.state.Editor.ChunkedBuffer.SetFileSize(int64(len(content)))
+					l.state.Editor.ChunkedBuffer.Insert(0, string(content))
+					// Also update the Buffer field for backward compatibility and for tests checking it.
+					l.state.Editor.Buffer = string(content)
+				} else {
+					// Fallback: populate the deprecated Buffer field
+					l.state.Editor.Buffer = string(content)
+				}
+			}
+		}
+	} else if res.TaskType == pool.TypeReadChunk {
+		if res.Success {
+			if chunk, ok := res.Data.([]byte); ok {
+				// Parse the task ID to get the chunk index.
+				// Format: "readchunk-<idx>-<basename>"
+				var chunkIdx int
+				fmt.Sscanf(res.TaskID, "readchunk-%d-", &chunkIdx)
+				if cb := l.state.Editor.ChunkedBuffer; cb != nil {
+					cb.chunks[chunkIdx] = chunk
+					log.Printf("Logic: Loaded chunk %d for %s (%d bytes)", chunkIdx, res.FilePath, len(chunk))
+				}
+			}
+		} else {
+			log.Printf("Logic: ReadChunkTask failed for %s: %v", res.FilePath, res.Error)
+		}
+	} else if res.TaskType == pool.TypeStatFile {
+		log.Printf("Logic: TypeStatFile result success=%v", res.Success)
+		if res.Success {
+			if stat, ok := res.Data.(*pool.FileStat); ok {
+				if l.state.Editor.ChunkedBuffer != nil {
+					l.state.Editor.ChunkedBuffer.SetFileSize(stat.Size)
+					// Load the chunk containing the cursor (position 0 at open)
+					l.state.Editor.ChunkedBuffer.LoadChunk(0)
+					// Prefetch adjacent chunks
+					l.state.Editor.ChunkedBuffer.Prefetch(0, 1)
+					// Dispatch line index build task
+					l.workerPool.Dispatch(pool.NewBuildLineIndexTask(l.state.Editor.Filename, l.mockFS))
+				}
+			}
+		}
+	} else if res.TaskType == pool.TypeBuildLineIndex {
+		log.Printf("Logic: TypeBuildLineIndex result success=%v", res.Success)
+		if res.Success {
+			if idx, ok := res.Data.(*types.LineIndex); ok {
+				if l.state.Editor.ChunkedBuffer != nil {
+					l.state.Editor.ChunkedBuffer.LineIndex = idx
+					log.Printf("Logic: LineIndex built with %d lines", idx.LineCount())
+				}
 			}
 		}
 	} else if res.TaskType == pool.TypeWriteFile {
@@ -313,10 +395,20 @@ func (l *Logic) FlushAll() {
 
 	for filename := range l.state.Editor.fileVersion {
 		if l.state.Editor.IsDirty() && l.state.Editor.Filename == filename {
-			// Trigger a synchronous write for the active dirty file
+			// Reconstruct full content from chunked buffer for saving
-			content := l.state.Editor.Buffer
+			var content []byte
+			if l.state.Editor.ChunkedBuffer != nil {
+				fullContent, err := l.state.Editor.ChunkedBuffer.FullContent()
+				if err != nil {
+					log.Printf("Error reconstructing full content for flush: %v", err)
+					return
+				}
+				content = []byte(fullContent)
+			} else {
+				content = []byte(l.state.Editor.Buffer)
+			}
 			// In a real app, this would be a blocking call to the FS
-			l.mockFS.WriteFileAtomic(filename, []byte(content))
+			l.mockFS.WriteFileAtomic(filename, content)
 			l.state.Editor.lastWriteVersion[filename] = l.state.Editor.fileVersion[filename]
 		}
 	}

internal/editor/state.go

@@ -1,12 +1,14 @@
 package editor
 
 import (
+	"fmt"
 	"log"
 	"sort"
 	"time"
 	"unicode/utf8"
 
 	"pad/internal/browser"
+	"pad/internal/io/pool/types"
 	"pad/internal/ui"
 	"gioui.org/io/key"
 )
@@ -46,7 +48,9 @@ const (
 
 // EditorState holds all editor-specific state.
 type EditorState struct {
-	Buffer           string
+	Buffer           string           // DEPRECATED: use ChunkedBuffer for large files
+	ChunkedBuffer    *ChunkedBuffer   // NEW: chunked file access for virtual scrolling
+	LineIndex        *types.LineIndex // NEW: line-to-byte-offset mapping
 	CursorPosition   int
 	GlyphLayout      ui.GlyphLayout
 	SelectionStart   int
@@ -60,6 +64,19 @@ type EditorState struct {
 	retryAttempts    map[string]int     // Added: tracks retry attempts
 }
 
+// GetBuffer returns the full buffer content, using ChunkedBuffer if available.
+func (e *EditorState) GetBuffer() string {
+	if e.ChunkedBuffer != nil {
+		fullContent, err := e.ChunkedBuffer.FullContent()
+		if err != nil {
+			log.Printf("Error getting full buffer content: %v", err)
+			return ""
+		}
+		return fullContent
+	}
+	return e.Buffer
+}
+
 // IsSaving returns true if a save is pending.
 func (e *EditorState) IsSaving() bool {
 	return e.saveTimer != nil
@@ -126,6 +143,8 @@ func NewState() *State {
 	}
 }
 
+
+
 func (s *State) SetScale(scale float32) {
 	s.scale = scale
 }
@@ -176,6 +195,7 @@ func ToggleWordWrap(data any) {
 // HandleScroll updates the editor scroll offset in response to a scroll gesture.
 // The delta is in pixels (from gesture.Scroll.Update). Convert to Dp.
 // Clamped to [0, MaxScroll] so content doesn't scroll past its ends.
+// Also evicts chunks far from the cursor to keep memory bounded.
 func HandleScroll(data any) {
 	delta := data.(int) // pixels
 	TheState.ScrollOffset += ui.ToDp(ui.Px(delta), TheState.scale)
@@ -185,6 +205,13 @@ func HandleScroll(data any) {
 	if TheState.ScrollOffset > TheState.MaxScroll {
 		TheState.ScrollOffset = TheState.MaxScroll
 	}
+	// Evict chunks far from the cursor to keep memory bounded.
+	// Only evict when the cursor is near the viewport (i.e., scrolled to top).
+	if cb := TheState.Editor.ChunkedBuffer; cb != nil {
+		cursorChunk := TheState.Editor.CursorPosition / cb.ChunkSize()
+		cb.EvictFarChunks(TheState.Editor.CursorPosition, 2)
+		_ = cursorChunk // suppress unused variable warning
+	}
 }
 
 // HandleBrowserScroll updates the browser scroll offset in response to a scroll gesture.
@@ -196,6 +223,9 @@ func HandleBrowserScroll(data any) {
 
 // GoToBrowser switches the app to the browser page.
 func GoToBrowser(data any) {
+	if TheLogic != nil {
+		TheLogic.FlushAll()
+	}
 	TheState.page = BrowserPage
 }
 
@@ -210,13 +240,6 @@ func OpenFile(data any) {
 	filename := data.(string)
 
 	// Dispatch a request to load the file
-	// We use a non-blocking approach to avoid deadlock. 
-	// The openFileChan is still used for communication, but we must
-	// ensure that the logic goroutine consumes it without holding locks 
-	// that prevent this function from returning.
-	// Actually, the best way to avoid deadlock here is to NOT send on a channel 
-	// directly, but instead update state in a way that the logic goroutine 
-	// picks up on the next cycle, or use a separate goroutine to send the task.
 	go func() {
 		TheLogic.openFileChan <- filename
 	}()
@@ -228,6 +251,16 @@ func OpenFile(data any) {
 	TheState.FocusedElementID = "editor_text" // Set focus to editor
 }
 
+// SetChunkedBuffer sets the chunked buffer for the current editor state.
+func SetChunkedBuffer(cb *ChunkedBuffer) {
+	TheState.Editor.ChunkedBuffer = cb
+}
+
+// SetLineIndex sets the line index for the current editor state.
+func SetLineIndex(li *types.LineIndex) {
+	TheState.Editor.LineIndex = li
+}
+
 // ToggleSortOrder cycles the browser sort mode through four modes.
 func ToggleSortOrder(data any) {
 	// Cycle through the 4 sort modes
@@ -248,10 +281,19 @@ func HandleCursorMove(delta int) {
 	if newPos < 0 {
 		newPos = 0
 	}
-	if newPos > len(TheState.Editor.Buffer) {
+	// Use ChunkedBuffer.FileLen() as the authoritative upper bound
-		newPos = len(TheState.Editor.Buffer)
+	var maxPos int
+	if cb := TheState.Editor.ChunkedBuffer; cb != nil {
+		maxPos = int(cb.FileLen())
 	}
-	log.Printf("HandleCursorMove: old=%d, new=%d", TheState.Editor.CursorPosition, newPos)
+	if maxPos == 0 {
+		// Fallback to Buffer length if ChunkedBuffer not yet initialized
+		maxPos = len(TheState.Editor.Buffer)
+	}
+	if newPos > maxPos {
+		newPos = maxPos
+	}
+	log.Printf("HandleCursorMove: old=%d, new=%d (max=%d)", TheState.Editor.CursorPosition, newPos, maxPos)
 	TheState.Editor.CursorPosition = newPos
 }
 
@@ -351,7 +393,19 @@ func HandleEnd() {
 	// Position after the last character of the line.
 	// If it's a newline, it's the newline itself.
 	start := layout.ByteOffsets[targetIdx]
-	r, size := utf8.DecodeRuneInString(TheState.Editor.Buffer[start:])
+	buf := TheState.Editor.ChunkedBuffer
+	var fileContent string
+	if buf != nil {
+		content, err := buf.FullContent()
+		if err != nil {
+			log.Printf("Error getting full content: %v", err)
+			return
+		}
+		fileContent = content
+	} else {
+		fileContent = TheState.Editor.Buffer
+	}
+	r, size := utf8.DecodeRuneInString(fileContent[start:])
 	if r == '\n' {
 		TheState.Editor.CursorPosition = start
 	} else {
@@ -466,20 +520,33 @@ func HandleVerticalCursorMove(up bool) {
 // HandleDelete removes the character after the cursor.
 func HandleDelete() {
 	pos := TheState.Editor.CursorPosition
-	buf := TheState.Editor.Buffer
+	buf := TheState.Editor.ChunkedBuffer
-	if pos >= len(buf) {
+	if buf != nil {
-		return
+		buf.Delete(pos, 1)
+		buf.UpdateLineIndexAfterEdit(pos, -1)
+	} else {
+		// Fallback to string-based editing for small files / no chunked buffer
+		str := TheState.Editor.Buffer
+		if pos >= len(str) {
+			return
+		}
+		TheState.Editor.Buffer = str[:pos] + str[pos+1:]
 	}
-	TheState.Editor.Buffer = buf[:pos] + buf[pos+1:]
 	markDirty()
 }
 
 // HandleInsert inserts a string at the current cursor position.
 func HandleInsert(s string) {
 	pos := TheState.Editor.CursorPosition
-	buf := TheState.Editor.Buffer
+	buf := TheState.Editor.ChunkedBuffer
-	// Simple string concatenation for now
+	if buf != nil {
-	TheState.Editor.Buffer = buf[:pos] + s + buf[pos:]
+		buf.Insert(pos, s)
+		buf.UpdateLineIndexAfterEdit(pos, len(s))
+	} else {
+		// Fallback to string-based editing for small files / no chunked buffer
+		str := TheState.Editor.Buffer
+		TheState.Editor.Buffer = str[:pos] + s + str[pos:]
+	}
 	TheState.Editor.CursorPosition += len(s)
 	markDirty()
 }
@@ -490,9 +557,15 @@ func HandleBackspace() {
 	if pos == 0 {
 		return
 	}
-	buf := TheState.Editor.Buffer
+	buf := TheState.Editor.ChunkedBuffer
-	// Simple string slice
+	if buf != nil {
-	TheState.Editor.Buffer = buf[:pos-1] + buf[pos:]
+		buf.Delete(pos-1, 1)
+		buf.UpdateLineIndexAfterEdit(pos-1, -1)
+	} else {
+		// Fallback to string-based editing for small files / no chunked buffer
+		str := TheState.Editor.Buffer
+		TheState.Editor.Buffer = str[:pos-1] + str[pos:]
+	}
 	TheState.Editor.CursorPosition--
 	markDirty()
 }
@@ -556,12 +629,22 @@ func EditorLayout(screenWidth, screenHeight ui.Dp, wordWrap bool) []ui.Element {
 		statusText = "Modified"
 	}
 
+	// Compute cursor position and file size for the bottom bar.
+	cursorPos := TheState.Editor.CursorPosition
+	var fileSize int
+	if cb := TheState.Editor.ChunkedBuffer; cb != nil {
+		fileSize = int(cb.FileLen())
+	} else {
+		fileSize = len(TheState.Editor.Buffer)
+	}
+	cursorPosText := fmt.Sprintf("%d / %d", cursorPos, fileSize)
+
 	bottomBar := ui.NewContainer(
 		bottomBarRegion,
 		ui.Color{R: 230, G: 230, B: 230, A: 255},
 		[]ui.Element{
 			ui.NewLabel(statusText, 12, ui.Region{X: 0, Y: ui.Dp(2), W: bottomBarW, H: ui.Dp(20)}, ui.AlignStart, "", nil),
-			ui.NewLabel("1024 / 50000", 12, ui.Region{X: 0, Y: ui.Dp(2), W: bottomBarW, H: ui.Dp(20)}, ui.AlignCenter, "", nil),
+			ui.NewLabel(cursorPosText, 12, ui.Region{X: 0, Y: ui.Dp(2), W: bottomBarW, H: ui.Dp(20)}, ui.AlignCenter, "", nil),
 			ui.NewLabel(wrapText, 12, ui.Region{X: 0, Y: ui.Dp(2), W: bottomBarW, H: ui.Dp(20)}, ui.AlignEnd, "wrap", []ui.Interaction{
 				{Gesture: ui.Tap, Handler: ToggleWordWrap},
 			}),
@@ -585,14 +668,49 @@ func EditorLayout(screenWidth, screenHeight ui.Dp, wordWrap bool) []ui.Element {
 	}
 	TheState.MaxScroll = maxScroll
 
+	// Compute visible content for virtual scrolling.
+	var visibleContent string
+	var visibleCursorPos int
+	var visibleScrollOffset ui.Dp
+
+	cb := TheState.Editor.ChunkedBuffer
+	if cb != nil {
+		viewportHeight := editorRegion.H
+		start, end := cb.VisibleByteRange(TheState.ScrollOffset, viewportHeight)
+
+		// Extract visible content from chunked buffer
+		visibleContent = cb.Content(start, end)
+
+		// Adjust cursor position to be relative to visibleContent
+		visibleCursorPos = TheState.Editor.CursorPosition - start
+		if visibleCursorPos < 0 {
+			visibleCursorPos = 0
+		}
+
+		// Adjust scroll offset to be relative to visibleContent origin
+		visibleScrollOffset = TheState.ScrollOffset
+
+		// Prefetch adjacent chunks for smooth scrolling.
+		// Only prefetch when the cursor chunk changes to avoid redundant work.
+		cursorChunk := visibleCursorPos / cb.ChunkSize()
+		if cursorChunk != cb.LastPrefetchedChunk() {
+			cb.Prefetch(cursorChunk, 1)
+		}
+	} else {
+		// Fallback: no chunked buffer, use full buffer (small files)
+		visibleContent = TheState.Editor.Buffer
+		visibleCursorPos = TheState.Editor.CursorPosition
+		visibleScrollOffset = TheState.ScrollOffset
+	}
+
 	// Add the TextField back in a way that passes the test.
 	editorElem := ui.NewTextField(
 		"editor_text",
-		TheState.Editor.Buffer,
+		visibleContent,
 		editorRegion,
 		editorRegion.W,
-		TheState.ScrollOffset,
+		visibleScrollOffset,
-		TheState.Editor.CursorPosition,
+		visibleCursorPos,
 		[]ui.Interaction{
 			{Gesture: ui.Scroll, Handler: HandleScroll},
 			{Gesture: ui.KeyDown, Handler: HandleKeyDown},
@@ -707,7 +825,19 @@ func SetCursorFromPoint(x, y float64) {
 	if rightmostIdx != -1 && x > rightmostX {
 		// Position at the end of the line content, before any trailing newline.
 		start := layout.ByteOffsets[rightmostIdx]
-		r, size := utf8.DecodeRuneInString(TheState.Editor.Buffer[start:])
+		buf := TheState.Editor.ChunkedBuffer
+		var fileContent string
+		if buf != nil {
+			content, err := buf.FullContent()
+			if err != nil {
+				log.Printf("Error getting full content: %v", err)
+				return
+			}
+			fileContent = content
+		} else {
+			fileContent = TheState.Editor.Buffer
+		}
+		r, size := utf8.DecodeRuneInString(fileContent[start:])
 		if r == '\n' {
 			TheState.Editor.CursorPosition = start
 		} else {

internal/io/pool/filesystem.go

@@ -9,6 +9,7 @@ type FileSystem interface {
 	DirExists(path string) bool
 	FileExists(path string) bool
 	ReadFile(path string) ([]byte, error)
+	ReadFileAt(path string, offset, size int) ([]byte, error)
 	WriteFile(path string, content []byte) error
 	WriteFileAtomic(path string, content []byte) error
 	DeleteFile(path string) error

internal/io/pool/mock/filesystem.go

@@ -181,6 +181,34 @@ func (fs *FileSystem) ReadFile(path string) ([]byte, error) {
 	return content, nil
 }
 
+// ReadFileAt reads a specific range of bytes from a file at the given path.
+func (fs *FileSystem) ReadFileAt(path string, offset, size int) ([]byte, error) {
+	fs.mu.Lock()
+
+	if fs.delay > 0 {
+		fs.mu.Unlock()
+		time.Sleep(fs.delay)
+		fs.mu.Lock()
+	}
+
+	f, ok := fs.files[path]
+	if !ok {
+		fs.mu.Unlock()
+		return nil, fmt.Errorf("file not found: %s", path)
+	}
+	if offset >= len(f.Content) {
+		fs.mu.Unlock()
+		return []byte{}, nil
+	}
+	end := offset + size
+	if end > len(f.Content) {
+		end = len(f.Content)
+	}
+	content := append([]byte(nil), f.Content[offset:end]...)
+	fs.mu.Unlock()
+	return content, nil
+}
+
 // WriteFile writes or replaces the content of a file at the given path.
 // This is the non-atomic path — use WriteFileAtomic for safety.
 func (fs *FileSystem) WriteFile(path string, content []byte) error {

internal/io/pool/real/filesystem.go

@@ -1,6 +1,7 @@
 package real
 
 import (
+	"io"
 	"os"
 	"path/filepath"
 	"pad/internal/io/pool/types"
@@ -37,6 +38,21 @@ func (fs *RealFileSystem) ReadFile(path string) ([]byte, error) {
 	return os.ReadFile(filepath.Join(fs.Root, path))
 }
 
+func (fs *RealFileSystem) ReadFileAt(path string, offset, size int) ([]byte, error) {
+	fullPath := filepath.Join(fs.Root, path)
+	f, err := os.Open(fullPath)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+	buf := make([]byte, size)
+	n, err := f.ReadAt(buf, int64(offset))
+	if err != nil && err != io.EOF {
+		return nil, err
+	}
+	return buf[:n], nil
+}
+
 func (fs *RealFileSystem) WriteFile(path string, content []byte) error {
 	// Simple write for backward compatibility if needed, 
 	// but defer to Atomic implementation.

internal/io/pool/task_test.go

@@ -307,8 +307,8 @@ func TestTask_TaskType_String(t *testing.T) {
 	}
 
 	for _, tt := range tests {
-		if string(tt.taskType) != tt.want {
+		if tt.taskType.String() != tt.want {
-			t.Errorf("TaskType %v = %q, want %q", tt.taskType, string(tt.taskType), tt.want)
+			t.Errorf("TaskType %v = %q, want %q", tt.taskType, tt.taskType.String(), tt.want)
 		}
 	}
 }

internal/io/pool/types/types.go

@@ -1,6 +1,9 @@
 package types
 
-import "os"
+import (
+	"fmt"
+	"os"
+)
 
 // DirEntry interface defines the structure for directory entries.
 type DirEntry interface {
@@ -8,3 +11,39 @@ type DirEntry interface {
 	IsDir() bool
 	Info() (os.FileInfo, error)
 }
+
+// LineIndex maps line numbers to byte offsets within a file.
+// Built as a low-priority background task; cached on disk.
+type LineIndex struct {
+	Offsets []int32 // byte offset of each line start (line 0 = 0)
+	Mtime   int64   // file mtime at index build time
+	Size    int64   // file size at index build time
+}
+
+// NewLineIndex creates a LineIndex from a list of byte offsets.
+func NewLineIndex(offsets []int32, mtime int64, size int64) *LineIndex {
+	return &LineIndex{
+		Offsets: offsets,
+		Mtime:   mtime,
+		Size:    size,
+	}
+}
+
+// LineCount returns the number of lines in the index.
+func (li *LineIndex) LineCount() int {
+	return len(li.Offsets)
+}
+
+// ByteOffset returns the byte offset of the given line number.
+// Returns 0 if the line number is out of range.
+func (li *LineIndex) ByteOffset(line int) int {
+	if line < 0 || line >= len(li.Offsets) {
+		return 0
+	}
+	return int(li.Offsets[line])
+}
+
+// String returns a string representation of the LineIndex.
+func (li *LineIndex) String() string {
+	return fmt.Sprintf("LineIndex{lines=%d, size=%d, mtime=%d}", len(li.Offsets), li.Size, li.Mtime)
+}

internal/test/e2e/edit_lifecycle_test.go

@@ -0,0 +1,90 @@
+package e2e_test
+
+import (
+	"testing"
+	"time"
+
+	"pad/internal/editor"
+	"pad/internal/test/e2e"
+	"pad/internal/ui"
+)
+
+func TestEditLifecycle(t *testing.T) {
+	h := e2e.NewHarnessWithDefaults()
+	defer h.Cleanup()
+
+	// 1. Go to browser page
+	editor.GoToBrowser(nil)
+	time.Sleep(200 * time.Millisecond)
+
+	// 2. Open File
+	filename := "/notes.txt"
+	editor.OpenFile(filename)
+
+	// Wait for file to load
+	success := false
+	for i := 0; i < 20; i++ {
+		if h.State().Editor.ChunkedBuffer != nil && h.State().Editor.ChunkedBuffer.FileLen() > 0 {
+			success = true
+			break
+		}
+		time.Sleep(100 * time.Millisecond)
+	}
+	if !success {
+		t.Fatal("Timed out waiting for file to load")
+	}
+
+	// 3. Edit File
+	// Original content of notes.txt is 198 bytes, let's append " UPDATED"
+	editor.HandleInsert(" UPDATED")
+	time.Sleep(100 * time.Millisecond)
+
+	// 4. Close File (Go back to browser)
+	// This will trigger FlushAll()
+	editor.GoToBrowser(nil)
+	time.Sleep(500 * time.Millisecond) // Ensure it had time to process close
+
+	// 5. Re-open File
+	editor.OpenFile(filename)
+	time.Sleep(1000 * time.Millisecond) // Wait for re-open and load
+
+	// 6. Verify Edits
+	fullContent, err := h.State().Editor.ChunkedBuffer.FullContent()
+	if err != nil {
+		t.Fatalf("Failed to reconstruct content: %v", err)
+	}
+	if len(fullContent) <= 198 {
+		t.Errorf("Content did not persist. Length: %d, expected > 198", len(fullContent))
+	}
+
+	// 7. Verify Size on Screen (check bottom bar label)
+	frame := e2e.GetLastFrame(h)
+	var sizeText string
+	for _, elem := range frame {
+		if container, ok := elem.(ui.Container); ok {
+			// The bottom bar is a container, look for labels inside
+			for _, child := range container.Children {
+				if label, ok := child.(ui.Label); ok {
+					// The cursor position text is in the middle, containing "/"
+					if len(label.Text) > 0 && label.Text[0] != '/' && contains(label.Text, "/") {
+						sizeText = label.Text
+					}
+				}
+			}
+		}
+	}
+
+	expectedText := "0 / 206" // 198 original + " UPDATED" (8) = 206, cursor at top
+	if sizeText != expectedText {
+		t.Errorf("Bottom bar size label mismatch: got %q, expected %q", sizeText, expectedText)
+	}
+}
+
+func contains(s, substr string) bool {
+	for i := 0; i < len(s); i++ {
+		if s[i] == substr[0] {
+			return true
+		}
+	}
+	return false
+}