diff --git a/internal/core/buffer_test.go b/internal/core/buffer_test.go
new file mode 100644
index 0000000..4620aa8
--- /dev/null
+++ b/internal/core/buffer_test.go
@@ -0,0 +1,302 @@
+package core
+
+import "testing"
+
+// --------------------------------------------------
+// Buffer Tests (generated by ClaudeCode)
+// --------------------------------------------------
+
+func TestBuffer_InsertLine(t *testing.T) {
+	t.Run("inserts at beginning", func(t *testing.T) {
+		buf := NewBufferBuilder().
+			WithLines([]string{"line 1", "line 2"}).
+			Build()
+
+		buf.InsertLine(0, "new line")
+
+		if buf.LineCount() != 3 {
+			t.Errorf("expected 3 lines, got %d", buf.LineCount())
+		}
+		if buf.Line(0) != "new line" {
+			t.Errorf("expected 'new line', got '%s'", buf.Line(0))
+		}
+		if buf.Line(1) != "line 1" {
+			t.Errorf("expected 'line 1' at index 1, got '%s'", buf.Line(1))
+		}
+	})
+
+	t.Run("inserts at end", func(t *testing.T) {
+		buf := NewBufferBuilder().
+			WithLines([]string{"line 1", "line 2"}).
+			Build()
+
+		buf.InsertLine(2, "new line")
+
+		if buf.LineCount() != 3 {
+			t.Errorf("expected 3 lines, got %d", buf.LineCount())
+		}
+		if buf.Line(2) != "new line" {
+			t.Errorf("expected 'new line' at end, got '%s'", buf.Line(2))
+		}
+	})
+
+	t.Run("inserts in middle", func(t *testing.T) {
+		buf := NewBufferBuilder().
+			WithLines([]string{"line 1", "line 3"}).
+			Build()
+
+		buf.InsertLine(1, "line 2")
+
+		if buf.LineCount() != 3 {
+			t.Errorf("expected 3 lines, got %d", buf.LineCount())
+		}
+		if buf.Line(1) != "line 2" {
+			t.Errorf("expected 'line 2' at index 1, got '%s'", buf.Line(1))
+		}
+	})
+
+	t.Run("handles empty buffer", func(t *testing.T) {
+		buf := NewBufferBuilder().Build()
+
+		buf.InsertLine(0, "first line")
+
+		if buf.LineCount() != 2 { // Original empty line + new line
+			t.Errorf("expected 2 lines, got %d", buf.LineCount())
+		}
+	})
+}
+
+func TestBuffer_DeleteLine(t *testing.T) {
+	t.Run("deletes from beginning", func(t *testing.T) {
+		buf := NewBufferBuilder().
+			WithLines([]string{"DELETE ME", "line 2", "line 3"}).
+			Build()
+
+		buf.DeleteLine(0)
+
+		if buf.LineCount() != 2 {
+			t.Errorf("expected 2 lines, got %d", buf.LineCount())
+		}
+		if buf.Line(0) != "line 2" {
+			t.Errorf("expected 'line 2' at index 0, got '%s'", buf.Line(0))
+		}
+	})
+
+	t.Run("deletes from middle", func(t *testing.T) {
+		buf := NewBufferBuilder().
+			WithLines([]string{"line 1", "DELETE ME", "line 3"}).
+			Build()
+
+		buf.DeleteLine(1)
+
+		if buf.LineCount() != 2 {
+			t.Errorf("expected 2 lines, got %d", buf.LineCount())
+		}
+		if buf.Line(1) != "line 3" {
+			t.Errorf("expected 'line 3' at index 1, got '%s'", buf.Line(1))
+		}
+	})
+
+	t.Run("deletes from end", func(t *testing.T) {
+		buf := NewBufferBuilder().
+			WithLines([]string{"line 1", "line 2", "DELETE ME"}).
+			Build()
+
+		buf.DeleteLine(2)
+
+		if buf.LineCount() != 2 {
+			t.Errorf("expected 2 lines, got %d", buf.LineCount())
+		}
+		if buf.Line(1) != "line 2" {
+			t.Errorf("expected 'line 2' at index 1, got '%s'", buf.Line(1))
+		}
+	})
+
+	t.Run("can delete all lines", func(t *testing.T) {
+		buf := NewBufferBuilder().
+			WithLines([]string{"only line"}).
+			Build()
+
+		buf.DeleteLine(0)
+
+		// Buffer allows being completely empty (0 lines)
+		if buf.LineCount() != 0 {
+			t.Errorf("expected 0 lines after deleting last line, got %d", buf.LineCount())
+		}
+	})
+}
+
+func TestBuffer_SetLine(t *testing.T) {
+	t.Run("updates existing line", func(t *testing.T) {
+		buf := NewBufferBuilder().
+			WithLines([]string{"old content"}).
+			Build()
+
+		buf.SetLine(0, "new content")
+
+		if buf.Line(0) != "new content" {
+			t.Errorf("expected 'new content', got '%s'", buf.Line(0))
+		}
+	})
+
+	t.Run("updates middle line", func(t *testing.T) {
+		buf := NewBufferBuilder().
+			WithLines([]string{"line 1", "old", "line 3"}).
+			Build()
+
+		buf.SetLine(1, "new")
+
+		if buf.Line(1) != "new" {
+			t.Errorf("expected 'new', got '%s'", buf.Line(1))
+		}
+		// Verify other lines unchanged
+		if buf.Line(0) != "line 1" {
+			t.Error("line 0 should be unchanged")
+		}
+		if buf.Line(2) != "line 3" {
+			t.Error("line 2 should be unchanged")
+		}
+	})
+
+	t.Run("can set to empty string", func(t *testing.T) {
+		buf := NewBufferBuilder().
+			WithLines([]string{"has content"}).
+			Build()
+
+		buf.SetLine(0, "")
+
+		if buf.Line(0) != "" {
+			t.Errorf("expected empty line, got '%s'", buf.Line(0))
+		}
+	})
+}
+
+func TestBuffer_LineCount(t *testing.T) {
+	t.Run("empty buffer has one line", func(t *testing.T) {
+		buf := NewBufferBuilder().Build()
+
+		if buf.LineCount() != 1 {
+			t.Errorf("expected 1 line in empty buffer, got %d", buf.LineCount())
+		}
+	})
+
+	t.Run("counts multiple lines", func(t *testing.T) {
+		buf := NewBufferBuilder().
+			WithLines([]string{"a", "b", "c", "d", "e"}).
+			Build()
+
+		if buf.LineCount() != 5 {
+			t.Errorf("expected 5 lines, got %d", buf.LineCount())
+		}
+	})
+
+	t.Run("counts after insertions", func(t *testing.T) {
+		buf := NewBufferBuilder().
+			WithLines([]string{"line 1"}).
+			Build()
+
+		buf.InsertLine(1, "line 2")
+		buf.InsertLine(2, "line 3")
+
+		if buf.LineCount() != 3 {
+			t.Errorf("expected 3 lines, got %d", buf.LineCount())
+		}
+	})
+
+	t.Run("counts after deletions", func(t *testing.T) {
+		buf := NewBufferBuilder().
+			WithLines([]string{"a", "b", "c", "d", "e"}).
+			Build()
+
+		buf.DeleteLine(2)
+		buf.DeleteLine(2)
+
+		if buf.LineCount() != 3 {
+			t.Errorf("expected 3 lines after 2 deletions, got %d", buf.LineCount())
+		}
+	})
+}
+
+func TestBuffer_Line(t *testing.T) {
+	t.Run("retrieves correct line", func(t *testing.T) {
+		buf := NewBufferBuilder().
+			WithLines([]string{"first", "second", "third"}).
+			Build()
+
+		if buf.Line(0) != "first" {
+			t.Errorf("expected 'first', got '%s'", buf.Line(0))
+		}
+		if buf.Line(1) != "second" {
+			t.Errorf("expected 'second', got '%s'", buf.Line(1))
+		}
+		if buf.Line(2) != "third" {
+			t.Errorf("expected 'third', got '%s'", buf.Line(2))
+		}
+	})
+
+	t.Run("handles special characters", func(t *testing.T) {
+		buf := NewBufferBuilder().
+			WithLines([]string{"hello\tworld", "foo\nbar"}).
+			Build()
+
+		if buf.Line(0) != "hello\tworld" {
+			t.Errorf("expected tabs preserved, got '%s'", buf.Line(0))
+		}
+	})
+
+	t.Run("handles unicode", func(t *testing.T) {
+		buf := NewBufferBuilder().
+			WithLines([]string{"hello 世界", "emoji 🎉"}).
+			Build()
+
+		if buf.Line(0) != "hello 世界" {
+			t.Errorf("expected unicode preserved, got '%s'", buf.Line(0))
+		}
+		if buf.Line(1) != "emoji 🎉" {
+			t.Errorf("expected emoji preserved, got '%s'", buf.Line(1))
+		}
+	})
+}
+
+func TestBufferBuilder(t *testing.T) {
+	t.Run("builds with default values", func(t *testing.T) {
+		buf := NewBufferBuilder().Build()
+
+		if buf.LineCount() != 1 {
+			t.Errorf("expected 1 empty line, got %d", buf.LineCount())
+		}
+		if buf.Filename != "" {
+			t.Errorf("expected empty filename, got '%s'", buf.Filename)
+		}
+	})
+
+	t.Run("builds with custom lines", func(t *testing.T) {
+		buf := NewBufferBuilder().
+			WithLines([]string{"line 1", "line 2"}).
+			Build()
+
+		if buf.LineCount() != 2 {
+			t.Errorf("expected 2 lines, got %d", buf.LineCount())
+		}
+	})
+
+	t.Run("builds with filename", func(t *testing.T) {
+		buf := NewBufferBuilder().
+			WithFilename("test.txt").
+			Build()
+
+		if buf.Filename != "test.txt" {
+			t.Errorf("expected filename 'test.txt', got '%s'", buf.Filename)
+		}
+	})
+
+	t.Run("builds with filetype", func(t *testing.T) {
+		buf := NewBufferBuilder().
+			WithFiletype("go").
+			Build()
+
+		if buf.Filetype != "go" {
+			t.Errorf("expected filetype 'go', got '%s'", buf.Filetype)
+		}
+	})
+}
diff --git a/internal/core/window_test.go b/internal/core/window_test.go
new file mode 100644
index 0000000..a755819
--- /dev/null
+++ b/internal/core/window_test.go
@@ -0,0 +1,493 @@
+package core
+
+import "testing"
+
+// --------------------------------------------------
+// Window Tests (generated by ClaudeCode)
+// --------------------------------------------------
+
+func TestWindow_SetCursorLine(t *testing.T) {
+	t.Run("clamps cursor below zero", func(t *testing.T) {
+		buf := NewBufferBuilder().
+			WithLines([]string{"line 1", "line 2", "line 3"}).
+			Build()
+		win := NewWindowBuilder().
+			WithBuffer(&buf).
+			Build()
+
+		win.SetCursorLine(-5)
+
+		if win.Cursor.Line != 0 {
+			t.Errorf("expected cursor at line 0, got %d", win.Cursor.Line)
+		}
+	})
+
+	t.Run("clamps cursor past end", func(t *testing.T) {
+		buf := NewBufferBuilder().
+			WithLines([]string{"line 1", "line 2", "line 3"}).
+			Build()
+		win := NewWindowBuilder().
+			WithBuffer(&buf).
+			Build()
+
+		win.SetCursorLine(999)
+
+		if win.Cursor.Line != 2 { // 3 lines, max index is 2
+			t.Errorf("expected cursor at line 2, got %d", win.Cursor.Line)
+		}
+	})
+
+	t.Run("allows valid position", func(t *testing.T) {
+		buf := NewBufferBuilder().
+			WithLines([]string{"line 1", "line 2", "line 3"}).
+			Build()
+		win := NewWindowBuilder().
+			WithBuffer(&buf).
+			Build()
+
+		win.SetCursorLine(1)
+
+		if win.Cursor.Line != 1 {
+			t.Errorf("expected cursor at line 1, got %d", win.Cursor.Line)
+		}
+	})
+
+	t.Run("handles empty buffer", func(t *testing.T) {
+		buf := NewBufferBuilder().Build()
+		win := NewWindowBuilder().
+			WithBuffer(&buf).
+			Build()
+
+		win.SetCursorLine(5)
+
+		if win.Cursor.Line != 0 {
+			t.Errorf("expected cursor at line 0 for empty buffer, got %d", win.Cursor.Line)
+		}
+	})
+}
+
+func TestWindow_SetCursorCol(t *testing.T) {
+	t.Run("clamps to line length", func(t *testing.T) {
+		buf := NewBufferBuilder().
+			WithLines([]string{"hello"}).
+			Build()
+		win := NewWindowBuilder().
+			WithBuffer(&buf).
+			Build()
+
+		win.SetCursorCol(999)
+
+		// "hello" is 5 chars, max col should be 5 (after last char for insert mode)
+		if win.Cursor.Col > 5 {
+			t.Errorf("expected cursor col <= 5, got %d", win.Cursor.Col)
+		}
+	})
+
+	t.Run("clamps below zero", func(t *testing.T) {
+		buf := NewBufferBuilder().
+			WithLines([]string{"hello"}).
+			Build()
+		win := NewWindowBuilder().
+			WithBuffer(&buf).
+			Build()
+
+		win.SetCursorCol(-10)
+
+		if win.Cursor.Col != 0 {
+			t.Errorf("expected cursor col 0, got %d", win.Cursor.Col)
+		}
+	})
+
+	t.Run("handles empty line", func(t *testing.T) {
+		buf := NewBufferBuilder().
+			WithLines([]string{""}).
+			Build()
+		win := NewWindowBuilder().
+			WithBuffer(&buf).
+			Build()
+
+		win.SetCursorCol(5)
+
+		if win.Cursor.Col != 0 {
+			t.Errorf("expected cursor at col 0 on empty line, got %d", win.Cursor.Col)
+		}
+	})
+
+	t.Run("allows cursor at end of line", func(t *testing.T) {
+		buf := NewBufferBuilder().
+			WithLines([]string{"hello"}).
+			Build()
+		win := NewWindowBuilder().
+			WithBuffer(&buf).
+			Build()
+
+		win.SetCursorCol(5) // After last char
+
+		if win.Cursor.Col != 5 {
+			t.Errorf("expected cursor at col 5, got %d", win.Cursor.Col)
+		}
+	})
+}
+
+func TestWindow_AdjustScroll(t *testing.T) {
+	t.Run("scrolls down when cursor goes below viewport", func(t *testing.T) {
+		// Create buffer with many lines
+		lines := make([]string, 100)
+		for i := range lines {
+			lines[i] = "line"
+		}
+		buf := NewBufferBuilder().WithLines(lines).Build()
+
+		win := NewWindowBuilder().
+			WithBuffer(&buf).
+			WithHeight(24).
+			Build()
+
+		// Start at top
+		win.SetCursorLine(0)
+		win.AdjustScroll()
+		initialScroll := win.ScrollY
+
+		// Move cursor way down
+		win.SetCursorLine(50)
+		win.AdjustScroll()
+
+		// Scroll should have increased
+		if win.ScrollY <= initialScroll {
+			t.Errorf("expected scroll to increase, was %d, now %d", initialScroll, win.ScrollY)
+		}
+
+		// Cursor should be visible
+		viewport := win.ViewportHeight()
+		if win.Cursor.Line < win.ScrollY || win.Cursor.Line >= win.ScrollY+viewport {
+			t.Errorf("cursor at %d not visible in scroll range [%d, %d)",
+				win.Cursor.Line, win.ScrollY, win.ScrollY+viewport)
+		}
+	})
+
+	t.Run("scrolls up when cursor goes above viewport", func(t *testing.T) {
+		lines := make([]string, 100)
+		for i := range lines {
+			lines[i] = "line"
+		}
+		buf := NewBufferBuilder().WithLines(lines).Build()
+
+		win := NewWindowBuilder().
+			WithBuffer(&buf).
+			WithHeight(24).
+			Build()
+
+		// Start at bottom
+		win.SetCursorLine(80)
+		win.AdjustScroll()
+		initialScroll := win.ScrollY
+
+		// Move cursor to top
+		win.SetCursorLine(5)
+		win.AdjustScroll()
+
+		// Scroll should have decreased
+		if win.ScrollY >= initialScroll {
+			t.Errorf("expected scroll to decrease, was %d, now %d", initialScroll, win.ScrollY)
+		}
+
+		// Cursor should be visible
+		viewport := win.ViewportHeight()
+		if win.Cursor.Line < win.ScrollY || win.Cursor.Line >= win.ScrollY+viewport {
+			t.Errorf("cursor at %d not visible in scroll range [%d, %d)",
+				win.Cursor.Line, win.ScrollY, win.ScrollY+viewport)
+		}
+	})
+
+	t.Run("respects scrolloff margin", func(t *testing.T) {
+		lines := make([]string, 100)
+		for i := range lines {
+			lines[i] = "line"
+		}
+		buf := NewBufferBuilder().WithLines(lines).Build()
+
+		win := NewWindowBuilder().
+			WithBuffer(&buf).
+			WithHeight(24).
+			Build()
+
+		// Set scrolloff to 5
+		opts := win.Options
+		opts.ScrollOff = 5
+		win.SetOptions(opts)
+
+		// Move to line 20 and adjust
+		win.SetCursorLine(20)
+		win.AdjustScroll()
+
+		viewport := win.ViewportHeight()
+		distFromTop := win.Cursor.Line - win.ScrollY
+		distFromBottom := (win.ScrollY + viewport - 1) - win.Cursor.Line
+
+		// At least one should respect scrolloff
+		if distFromTop < opts.ScrollOff && distFromBottom < opts.ScrollOff {
+			t.Errorf("scrolloff %d not respected: top=%d, bottom=%d",
+				opts.ScrollOff, distFromTop, distFromBottom)
+		}
+	})
+
+	t.Run("handles scrolloff larger than half viewport", func(t *testing.T) {
+		lines := make([]string, 100)
+		for i := range lines {
+			lines[i] = "line"
+		}
+		buf := NewBufferBuilder().WithLines(lines).Build()
+
+		win := NewWindowBuilder().
+			WithBuffer(&buf).
+			WithHeight(24).
+			Build()
+
+		// Set scrolloff larger than half viewport
+		opts := win.Options
+		opts.ScrollOff = 999
+		win.SetOptions(opts)
+
+		win.SetCursorLine(50)
+		win.AdjustScroll()
+
+		// Should not panic or error
+		viewport := win.ViewportHeight()
+		if win.Cursor.Line < win.ScrollY || win.Cursor.Line >= win.ScrollY+viewport {
+			t.Error("cursor should still be visible with large scrolloff")
+		}
+	})
+
+	t.Run("handles small viewport", func(t *testing.T) {
+		lines := make([]string, 100)
+		for i := range lines {
+			lines[i] = "line"
+		}
+		buf := NewBufferBuilder().WithLines(lines).Build()
+
+		win := NewWindowBuilder().
+			WithBuffer(&buf).
+			WithHeight(5). // Very small
+			Build()
+
+		win.SetCursorLine(50)
+		win.AdjustScroll()
+
+		// Should not panic
+		viewport := win.ViewportHeight()
+		if viewport > 0 && (win.Cursor.Line < win.ScrollY || win.Cursor.Line >= win.ScrollY+viewport) {
+			t.Error("cursor should be visible in small viewport")
+		}
+	})
+}
+
+func TestWindow_ViewportHeight(t *testing.T) {
+	t.Run("calculates viewport height correctly", func(t *testing.T) {
+		buf := NewBufferBuilder().Build()
+		win := NewWindowBuilder().
+			WithBuffer(&buf).
+			WithHeight(24).
+			Build()
+
+		// Height - 2 (status bar + command bar)
+		expected := 22
+		if win.ViewportHeight() != expected {
+			t.Errorf("expected viewport height %d, got %d", expected, win.ViewportHeight())
+		}
+	})
+
+	t.Run("handles small window", func(t *testing.T) {
+		buf := NewBufferBuilder().Build()
+		win := NewWindowBuilder().
+			WithBuffer(&buf).
+			WithHeight(3).
+			Build()
+
+		// 3 - 2 = 1
+		expected := 1
+		if win.ViewportHeight() != expected {
+			t.Errorf("expected viewport height %d, got %d", expected, win.ViewportHeight())
+		}
+	})
+
+	t.Run("handles zero height", func(t *testing.T) {
+		buf := NewBufferBuilder().Build()
+		win := NewWindowBuilder().
+			WithBuffer(&buf).
+			WithHeight(0).
+			Build()
+
+		// With height 0, viewport is 0 - 2 (status + command bars) = -2
+		// This is an edge case that shouldn't occur in practice, but shouldn't panic
+		result := win.ViewportHeight()
+		expected := -2
+		if result != expected {
+			t.Errorf("expected viewport height %d for zero height window, got %d", expected, result)
+		}
+	})
+}
+
+func TestWindow_SetOptions(t *testing.T) {
+	t.Run("updates options", func(t *testing.T) {
+		buf := NewBufferBuilder().Build()
+		win := NewWindowBuilder().
+			WithBuffer(&buf).
+			Build()
+
+		newOpts := WinOptions{
+			Number:         false,
+			RelativeNumber: false,
+			GutterSize:     10,
+			ScrollOff:      3,
+		}
+
+		win.SetOptions(newOpts)
+
+		if win.Options.Number != false {
+			t.Error("expected Number to be false")
+		}
+		if win.Options.RelativeNumber != false {
+			t.Error("expected RelativeNumber to be false")
+		}
+		if win.Options.GutterSize != 10 {
+			t.Errorf("expected GutterSize 10, got %d", win.Options.GutterSize)
+		}
+		if win.Options.ScrollOff != 3 {
+			t.Errorf("expected ScrollOff 3, got %d", win.Options.ScrollOff)
+		}
+	})
+
+	t.Run("can toggle individual options", func(t *testing.T) {
+		buf := NewBufferBuilder().Build()
+		win := NewWindowBuilder().
+			WithBuffer(&buf).
+			Build()
+
+		// Get current options
+		opts := win.Options
+		originalNumber := opts.Number
+
+		// Toggle number
+		opts.Number = !opts.Number
+		win.SetOptions(opts)
+
+		if win.Options.Number == originalNumber {
+			t.Error("Number option should have toggled")
+		}
+	})
+}
+
+func TestWindow_SetAnchor(t *testing.T) {
+	t.Run("sets anchor line", func(t *testing.T) {
+		buf := NewBufferBuilder().
+			WithLines([]string{"a", "b", "c"}).
+			Build()
+		win := NewWindowBuilder().
+			WithBuffer(&buf).
+			Build()
+
+		win.SetAnchorLine(2)
+
+		if win.Anchor.Line != 2 {
+			t.Errorf("expected anchor line 2, got %d", win.Anchor.Line)
+		}
+	})
+
+	t.Run("sets anchor col", func(t *testing.T) {
+		buf := NewBufferBuilder().
+			WithLines([]string{"hello world"}).
+			Build()
+		win := NewWindowBuilder().
+			WithBuffer(&buf).
+			Build()
+
+		win.SetAnchorCol(5)
+
+		if win.Anchor.Col != 5 {
+			t.Errorf("expected anchor col 5, got %d", win.Anchor.Col)
+		}
+	})
+}
+
+func TestWindow_SetDimensions(t *testing.T) {
+	t.Run("updates width and height", func(t *testing.T) {
+		buf := NewBufferBuilder().Build()
+		win := NewWindowBuilder().
+			WithBuffer(&buf).
+			Build()
+
+		win.SetDimensions(100, 50)
+
+		if win.Width != 100 {
+			t.Errorf("expected width 100, got %d", win.Width)
+		}
+		if win.Height != 50 {
+			t.Errorf("expected height 50, got %d", win.Height)
+		}
+	})
+}
+
+func TestWindowBuilder(t *testing.T) {
+	t.Run("builds with defaults", func(t *testing.T) {
+		buf := NewBufferBuilder().Build()
+		win := NewWindowBuilder().
+			WithBuffer(&buf).
+			Build()
+
+		// Should have default options
+		if win.Options.Number != true {
+			t.Error("expected default Number to be true")
+		}
+		if win.Options.RelativeNumber != true {
+			t.Error("expected default RelativeNumber to be true")
+		}
+		if win.Options.ScrollOff != 8 {
+			t.Errorf("expected default ScrollOff 8, got %d", win.Options.ScrollOff)
+		}
+		if win.Options.GutterSize != 5 {
+			t.Errorf("expected default GutterSize 5, got %d", win.Options.GutterSize)
+		}
+	})
+
+	t.Run("builds with custom cursor position", func(t *testing.T) {
+		buf := NewBufferBuilder().
+			WithLines([]string{"a", "b", "c"}).
+			Build()
+		win := NewWindowBuilder().
+			WithBuffer(&buf).
+			WithCursorPos(2, 0).
+			Build()
+
+		if win.Cursor.Line != 2 {
+			t.Errorf("expected cursor line 2, got %d", win.Cursor.Line)
+		}
+		if win.Cursor.Col != 0 {
+			t.Errorf("expected cursor col 0, got %d", win.Cursor.Col)
+		}
+	})
+
+	t.Run("builds with custom dimensions", func(t *testing.T) {
+		buf := NewBufferBuilder().Build()
+		win := NewWindowBuilder().
+			WithBuffer(&buf).
+			WithDimensions(120, 40).
+			Build()
+
+		if win.Width != 120 {
+			t.Errorf("expected width 120, got %d", win.Width)
+		}
+		if win.Height != 40 {
+			t.Errorf("expected height 40, got %d", win.Height)
+		}
+	})
+
+	t.Run("assigns unique IDs", func(t *testing.T) {
+		buf := NewBufferBuilder().Build()
+		win1 := NewWindowBuilder().WithBuffer(&buf).Build()
+		win2 := NewWindowBuilder().WithBuffer(&buf).Build()
+
+		if win1.Id == win2.Id {
+			t.Errorf("expected unique IDs, both were %d", win1.Id)
+		}
+	})
+}