fix: cursor scroll on line wrap, Unicode search backspace, and increase test coverage

Fix editor Left/Right arrow keys not calling scrollToCursor() when
wrapping across lines, causing the cursor to go off-screen. Fix viewer
search backspace corrupting multi-byte Unicode characters by using
rune-based truncation. Remove dead code in NewBuffer.

Add 41 new tests covering edge cases, boundary conditions, and
previously untested code paths. Coverage: 74.2% -> 91.3%.

Author: tirthpatell

internal/editor/buffer.go

@@ -12,9 +12,6 @@ type Buffer struct {
 
 func NewBuffer(content string) *Buffer {
 	lines := strings.Split(content, "\n")
-	if len(lines) == 0 {
-		lines = []string{""}
-	}
 	return &Buffer{lines: lines}
 }
 

internal/editor/buffer_test.go

@@ -107,3 +107,118 @@ func TestBuffer_Modified(t *testing.T) {
 		t.Fatal("buffer should be modified after insert")
 	}
 }
+
+func TestBuffer_GetLine_OutOfBounds(t *testing.T) {
+	b := NewBuffer("hello")
+	if got := b.GetLine(-1); got != "" {
+		t.Fatalf("expected empty string for negative row, got %q", got)
+	}
+	if got := b.GetLine(5); got != "" {
+		t.Fatalf("expected empty string for row beyond end, got %q", got)
+	}
+}
+
+func TestBuffer_LineLen_OutOfBounds(t *testing.T) {
+	b := NewBuffer("hello")
+	if got := b.LineLen(-1); got != 0 {
+		t.Fatalf("expected 0 for negative row, got %d", got)
+	}
+	if got := b.LineLen(5); got != 0 {
+		t.Fatalf("expected 0 for row beyond end, got %d", got)
+	}
+}
+
+func TestBuffer_InsertChar_OutOfBounds(t *testing.T) {
+	b := NewBuffer("hello")
+	// Invalid row — should be no-op
+	b.InsertChar(-1, 0, 'x')
+	b.InsertChar(5, 0, 'x')
+	if got := b.GetLine(0); got != "hello" {
+		t.Fatalf("expected unchanged line, got %q", got)
+	}
+	// Col beyond line length — should clamp
+	b.InsertChar(0, 100, '!')
+	if got := b.GetLine(0); got != "hello!" {
+		t.Fatalf("expected 'hello!' with clamped col, got %q", got)
+	}
+}
+
+func TestBuffer_DeleteChar_OutOfBounds(t *testing.T) {
+	b := NewBuffer("hello")
+	// Invalid row
+	b.DeleteChar(-1, 0)
+	b.DeleteChar(5, 0)
+	// Invalid col
+	b.DeleteChar(0, -1)
+	b.DeleteChar(0, 10)
+	if got := b.GetLine(0); got != "hello" {
+		t.Fatalf("expected unchanged line, got %q", got)
+	}
+}
+
+func TestBuffer_Backspace_AtStart(t *testing.T) {
+	b := NewBuffer("hello")
+	b.Backspace(0, 0) // at start of line — should be no-op
+	if got := b.GetLine(0); got != "hello" {
+		t.Fatalf("expected unchanged line, got %q", got)
+	}
+}
+
+func TestBuffer_InsertNewline_OutOfBounds(t *testing.T) {
+	b := NewBuffer("hello")
+	b.InsertNewline(-1, 0)
+	b.InsertNewline(5, 0)
+	if b.LineCount() != 1 {
+		t.Fatalf("expected 1 line after out-of-bounds newline inserts, got %d", b.LineCount())
+	}
+}
+
+func TestBuffer_InsertNewline_ColClamp(t *testing.T) {
+	b := NewBuffer("hello")
+	b.InsertNewline(0, 100) // col beyond length, should clamp
+	if b.LineCount() != 2 {
+		t.Fatalf("expected 2 lines, got %d", b.LineCount())
+	}
+	if got := b.GetLine(0); got != "hello" {
+		t.Fatalf("expected 'hello', got %q", got)
+	}
+	if got := b.GetLine(1); got != "" {
+		t.Fatalf("expected empty second line, got %q", got)
+	}
+}
+
+func TestBuffer_JoinLines_OutOfBounds(t *testing.T) {
+	b := NewBuffer("hello\nworld")
+	// row 0 — can't join with row above
+	col := b.JoinLines(0)
+	if col != 0 {
+		t.Fatalf("expected 0 for out-of-bounds join, got %d", col)
+	}
+	// row beyond end
+	col = b.JoinLines(5)
+	if col != 0 {
+		t.Fatalf("expected 0 for out-of-bounds join, got %d", col)
+	}
+	if b.LineCount() != 2 {
+		t.Fatalf("expected unchanged line count 2, got %d", b.LineCount())
+	}
+}
+
+func TestBuffer_LineLen_Unicode(t *testing.T) {
+	b := NewBuffer("héllo 世界")
+	if got := b.LineLen(0); got != 8 {
+		t.Fatalf("expected 8 runes, got %d", got)
+	}
+}
+
+func TestBuffer_ResetModified(t *testing.T) {
+	b := NewBuffer("hello")
+	b.InsertChar(0, 0, 'x')
+	if !b.Modified() {
+		t.Fatal("expected modified after insert")
+	}
+	b.ResetModified()
+	if b.Modified() {
+		t.Fatal("expected not modified after reset")
+	}
+}

internal/editor/editor.go

@@ -125,6 +125,7 @@ func (m Model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 			} else if m.cursorRow > 0 {
 				m.cursorRow--
 				m.cursorCol = m.buffer.LineLen(m.cursorRow)
+				m.scrollToCursor()
 			}
 			return m, nil
 
@@ -135,6 +136,7 @@ func (m Model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 			} else if m.cursorRow < m.buffer.LineCount()-1 {
 				m.cursorRow++
 				m.cursorCol = 0
+				m.scrollToCursor()
 			}
 			return m, nil