Fix redundant file loading and buffer insertion bugs

- Fix ChunkedBuffer.Insert to always update file length.
- Fix UpdateLineIndexAfterEdit to not shift the first line offset.
- Remove redundant ReadFileTask dispatch in HandleBrowserTap.
- Add end-to-end test for typing at start of buffer.

internal/browser/handlers.go

@@ -5,7 +5,6 @@ import (
 	"path/filepath"
 	"strings"
 
-	"pad/internal/io/pool"
 	"pad/internal/ui"
 )
 
@@ -98,8 +97,8 @@ func HandleBrowserTap(bm *BrowserManager, s *BrowserState, index int) {
 			ui.OpenFile(entry.Path)
 		}
 		// Dispatch ReadFileTask to the worker pool
-		task := pool.NewReadFileTask(entry.Path, bm.fs)
-		fmt.Printf("HandleBrowserTap: Dispatching ReadFileTask for %s\n", entry.Path)
-		bm.workerPool.Dispatch(task)
+		// task := pool.NewReadFileTask(entry.Path, bm.fs)
+		// fmt.Printf("HandleBrowserTap: Dispatching ReadFileTask for %s\n", entry.Path)
+		// bm.workerPool.Dispatch(task)
 	}
 }

internal/editor/chunked_buffer.go

@@ -59,8 +59,10 @@ func NewChunkedBuffer(filename string, chunkSize int, fs pool.FileSystem, basePa
 
 // SetFileSize sets the total file length.
 func (cb *ChunkedBuffer) SetFileSize(length int64) {
+	if !cb.dirty {
 		cb.fileLen = length
 	}
+}
 
 // FileLen returns the total file length.
 func (cb *ChunkedBuffer) FileLen() int64 {
@@ -288,10 +290,7 @@ func (cb *ChunkedBuffer) Insert(pos int, text string) {
 	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.fileLen += int64(len(text))
 
 	cb.dirty = true
 	cb.dirtyChunks[chunkIdx] = true
@@ -538,6 +537,9 @@ func (cb *ChunkedBuffer) UpdateLineIndexAfterEdit(editPos int, offsetShift int)
 	idx := sort.Search(len(cb.LineIndex.Offsets), func(i int) bool {
 		return int(cb.LineIndex.Offsets[i]) >= editPos
 	})
+	if idx == 0 {
+		idx = 1
+	}
 	for i := idx; i < len(cb.LineIndex.Offsets); i++ {
 		cb.LineIndex.Offsets[i] += int32(offsetShift)
 	}

internal/test/e2e/type_at_start_test.go

@@ -0,0 +1,154 @@
+package e2e_test
+
+import (
+	"strings"
+	"testing"
+	"time"
+
+	"pad/internal/editor"
+	"pad/internal/test/e2e"
+	"pad/internal/ui"
+)
+
+// TestTypeAtStartOfBuffer verifies that typing at cursor position 0 inserts
+// characters at the start of the buffer without deleting characters from the end,
+// and that they actually appear on screen (in the visible TextField).
+func TestTypeAtStartOfBuffer(t *testing.T) {
+	h := e2e.NewHarnessWithDefaults()
+	h.Run()
+	defer h.Cleanup()
+
+	// 1. Navigate to browser, then open a file.
+	editor.GoToBrowser(nil)
+	time.Sleep(200 * time.Millisecond)
+
+	filename := "/notes.txt"
+	editor.OpenFile(filename)
+
+	// 2. Wait for the file content to be loaded into the chunked buffer and line index built.
+	loaded := false
+	for i := 0; i < 30; i++ {
+		cb := h.State().Editor.ChunkedBuffer
+		if cb != nil && cb.FileLen() > 0 && cb.LineIndex != nil {
+			loaded = true
+			break
+		}
+		time.Sleep(100 * time.Millisecond)
+	}
+	if !loaded {
+		t.Fatal("timed out waiting for file to load and line index to build")
+	}
+
+	// Record original file length and content
+	cb := h.State().Editor.ChunkedBuffer
+	originalLen := int(cb.FileLen())
+	originalContent, err := cb.FullContent()
+	if err != nil {
+		t.Fatalf("failed to read original content: %v", err)
+	}
+	originalTail := originalContent[max(0, originalLen-20):]
+
+	// Confirm cursor is at position 0 (top of file).
+	if h.State().Editor.CursorPosition != 0 {
+		t.Fatalf("expected cursor at 0, got %d", h.State().Editor.CursorPosition)
+	}
+
+	// Get initial frame to verify the text field content before typing.
+	initialFrame := e2e.GetLastFrame(h)
+	var initialTextField *ui.TextField
+	for _, elem := range initialFrame {
+		if tf, ok := elem.(ui.TextField); ok && tf.ID() == "editor_text" {
+			initialTextField = &tf
+			break
+		}
+	}
+	if initialTextField == nil {
+		t.Fatal("could not find editor text field in initial frame")
+	}
+	if initialTextField.Value != originalContent {
+		t.Fatalf("expected initial text field to contain full original content, got length %d vs %d",
+			len(initialTextField.Value), len(originalContent))
+	}
+
+	// 3. Type some text at the start of the buffer.
+	typedText := "AAA "
+	h.SendInput([]ui.InputEvent{
+		{
+			Handler: func(data any) {
+				editor.HandleInsert(typedText)
+			},
+			Data: nil,
+		},
+	})
+
+	// 4. Wait for a new frame to be captured.
+	time.Sleep(200 * time.Millisecond)
+
+	// 5. Assert on the buffer content.
+	newContent, err := h.State().Editor.ChunkedBuffer.FullContent()
+	if err != nil {
+		t.Fatalf("failed to read content after insert: %v", err)
+	}
+
+	// 5a. The typed text must be at the very start of full content.
+	if !strings.HasPrefix(newContent, typedText) {
+		t.Errorf("expected full content to start with %q, got prefix %q",
+			typedText, newContent[:min(len(typedText), len(newContent))])
+	}
+
+	// 5b. The original tail must still be present at the end of full content.
+	if !strings.HasSuffix(newContent, originalTail) {
+		t.Errorf("expected full content to end with original tail %q, got suffix %q",
+			originalTail, newContent[len(newContent)-len(originalTail):])
+	}
+
+	// 5c. The total length of full content should be originalLen + len(typedText).
+	expectedLen := originalLen + len(typedText)
+	if len(newContent) != expectedLen {
+		t.Errorf("expected full content length %d, got %d", expectedLen, len(newContent))
+	}
+
+	// 6. Assert on the RENDERED TextField (visible content on screen).
+	finalFrame := e2e.GetLastFrame(h)
+	var finalTextField *ui.TextField
+	for _, elem := range finalFrame {
+		if tf, ok := elem.(ui.TextField); ok && tf.ID() == "editor_text" {
+			finalTextField = &tf
+			break
+		}
+	}
+	if finalTextField == nil {
+		t.Fatal("could not find editor text field in final frame")
+	}
+
+	// 6a. The typed characters must appear at the start of the text field.
+	if !strings.HasPrefix(finalTextField.Value, typedText) {
+		t.Errorf("BUG: typed characters do not appear at the start of the text field. Expected prefix %q, got %q",
+			typedText, finalTextField.Value[:min(len(typedText), len(finalTextField.Value))])
+	}
+
+	// 6b. The end of the file must not be truncated in the text field.
+	if !strings.HasSuffix(finalTextField.Value, originalTail) {
+		t.Errorf("BUG: characters at the end of the text field disappeared. Expected suffix %q, got %q",
+			originalTail, finalTextField.Value[max(0, len(finalTextField.Value)-len(originalTail)):])
+	}
+
+	// 6c. The visible text field length must be correct.
+	if len(finalTextField.Value) != expectedLen {
+		t.Errorf("expected visible text field length %d, got %d", expectedLen, len(finalTextField.Value))
+	}
+}
+
+func max(a, b int) int {
+	if a > b {
+		return a
+	}
+	return b
+}
+
+func min(a, b int) int {
+	if a < b {
+		return a
+	}
+	return b
+}