refactor(display)!: remove deprecated backends, simplify protocol, and add BorderMode/UI rendering

- Remove SixelDisplay and KittyDisplay backends (unmaintained)
- Simplify Display protocol: reduce docstring noise, emphasize duck typing
- Add BorderMode enum (OFF, SIMPLE, UI) for flexible border rendering
- Rename render_border to _render_simple_border
- Add render_ui_panel() to compose main viewport with right-side UI panel
- Add new render_border() dispatcher supporting BorderMode
- Update __all__ to expose BorderMode, render_ui_panel, PygameDisplay
- Clean up DisplayRegistry: remove deprecated method docstrings
- Update tests: remove SixelDisplay import, assert sixel not in registry
- Add TODO comment to WebSocket backend about streaming improvements

This is a breaking change (removal of backends) but enables cleaner architecture and interactive UI panel.

Closes #13, #21

engine/display/__init__.py

@@ -5,102 +5,58 @@ Allows swapping output backends via the Display protocol.
 Supports auto-discovery of display backends.
 """
 
+from enum import Enum, auto
 from typing import Protocol
 
-from engine.display.backends.kitty import KittyDisplay
+# Optional backend - requires moderngl package
+try:
+    from engine.display.backends.moderngl import ModernGLDisplay
+
+    _MODERNGL_AVAILABLE = True
+except ImportError:
+    ModernGLDisplay = None
+    _MODERNGL_AVAILABLE = False
+
 from engine.display.backends.multi import MultiDisplay
 from engine.display.backends.null import NullDisplay
 from engine.display.backends.pygame import PygameDisplay
-from engine.display.backends.sixel import SixelDisplay
 from engine.display.backends.terminal import TerminalDisplay
 from engine.display.backends.websocket import WebSocketDisplay
 
 
+class BorderMode(Enum):
+    """Border rendering modes for displays."""
+
+    OFF = auto()  # No border
+    SIMPLE = auto()  # Traditional border with FPS/frame time
+    UI = auto()  # Right-side UI panel with interactive controls
+
+
 class Display(Protocol):
     """Protocol for display backends.
 
-    All display backends must implement:
+    Required attributes:
-    - width, height: Terminal dimensions
+    - width: int
-    - init(width, height, reuse=False): Initialize the display
+    - height: int
-    - show(buffer): Render buffer to display
-    - clear(): Clear the display
-    - cleanup(): Shutdown the display
 
-    Optional methods for keyboard input:
+    Required methods (duck typing - actual signatures may vary):
-    - is_quit_requested(): Returns True if user pressed Ctrl+C/Q or Escape
+    - init(width, height, reuse=False)
-    - clear_quit_request(): Clears the quit request flag
+    - show(buffer, border=False)
+    - clear()
+    - cleanup()
+    - get_dimensions() -> (width, height)
 
-    The reuse flag allows attaching to an existing display instance
+    Optional attributes (for UI mode):
-    rather than creating a new window/connection.
+    - ui_panel: UIPanel instance (set by app when border=UI)
 
-    Keyboard input support by backend:
+    Optional methods:
-    - terminal: No native input (relies on signal handler for Ctrl+C)
+    - is_quit_requested() -> bool
-    - pygame: Supports Ctrl+C, Ctrl+Q, Escape for graceful shutdown
+    - clear_quit_request() -> None
-    - websocket: No native input (relies on signal handler for Ctrl+C)
-    - sixel: No native input (relies on signal handler for Ctrl+C)
-    - null: No native input
-    - kitty: Supports Ctrl+C, Ctrl+Q, Escape (via pygame-like handling)
     """
 
     width: int
     height: int
 
-    def init(self, width: int, height: int, reuse: bool = False) -> None:
-        """Initialize display with dimensions.
-
-        Args:
-            width: Terminal width in characters
-            height: Terminal height in rows
-            reuse: If True, attach to existing display instead of creating new
-        """
-        ...
-
-    def show(self, buffer: list[str], border: bool = False) -> None:
-        """Show buffer on display.
-
-        Args:
-            buffer: Buffer to display
-            border: If True, render border around buffer (default False)
-        """
-        ...
-
-    def clear(self) -> None:
-        """Clear display."""
-        ...
-
-    def cleanup(self) -> None:
-        """Shutdown display."""
-        ...
-
-    def get_dimensions(self) -> tuple[int, int]:
-        """Get current terminal dimensions.
-
-        Returns:
-            (width, height) in character cells
-
-        This method is called after show() to check if the display
-        was resized. The main loop should compare this to the current
-        viewport dimensions and update accordingly.
-        """
-        ...
-
-    def is_quit_requested(self) -> bool:
-        """Check if user requested quit (Ctrl+C, Ctrl+Q, or Escape).
-
-        Returns:
-            True if quit was requested, False otherwise
-
-        Optional method - only implemented by backends that support keyboard input.
-        """
-        ...
-
-    def clear_quit_request(self) -> None:
-        """Clear the quit request flag.
-
-        Optional method - only implemented by backends that support keyboard input.
-        """
-        ...
-
 
 class DisplayRegistry:
     """Registry for display backends with auto-discovery."""
@@ -110,22 +66,18 @@ class DisplayRegistry:
 
     @classmethod
     def register(cls, name: str, backend_class: type[Display]) -> None:
-        """Register a display backend."""
         cls._backends[name.lower()] = backend_class
 
     @classmethod
     def get(cls, name: str) -> type[Display] | None:
-        """Get a display backend class by name."""
         return cls._backends.get(name.lower())
 
     @classmethod
     def list_backends(cls) -> list[str]:
-        """List all available display backend names."""
         return list(cls._backends.keys())
 
     @classmethod
     def create(cls, name: str, **kwargs) -> Display | None:
-        """Create a display instance by name."""
         cls.initialize()
         backend_class = cls.get(name)
         if backend_class:
@@ -134,31 +86,18 @@ class DisplayRegistry:
 
     @classmethod
     def initialize(cls) -> None:
-        """Initialize and register all built-in backends."""
         if cls._initialized:
             return
-
         cls.register("terminal", TerminalDisplay)
         cls.register("null", NullDisplay)
         cls.register("websocket", WebSocketDisplay)
-        cls.register("sixel", SixelDisplay)
-        cls.register("kitty", KittyDisplay)
         cls.register("pygame", PygameDisplay)
-
+        if _MODERNGL_AVAILABLE:
+            cls.register("moderngl", ModernGLDisplay)  # type: ignore[arg-type]
         cls._initialized = True
 
     @classmethod
-    def create_multi(cls, names: list[str]) -> "Display | None":
+    def create_multi(cls, names: list[str]) -> MultiDisplay | None:
-        """Create a MultiDisplay from a list of backend names.
-
-        Args:
-            names: List of display backend names (e.g., ["terminal", "pygame"])
-
-        Returns:
-            MultiDisplay instance or None if any backend fails
-        """
-        from engine.display.backends.multi import MultiDisplay
-
         displays = []
         for name in names:
             backend = cls.create(name)
@@ -166,10 +105,8 @@ class DisplayRegistry:
                 displays.append(backend)
             else:
                 return None
-
         if not displays:
             return None
-
         return MultiDisplay(displays)
 
 
@@ -190,44 +127,28 @@ def _strip_ansi(s: str) -> str:
     return re.sub(r"\x1b\[[0-9;]*[a-zA-Z]", "", s)
 
 
-def render_border(
+def _render_simple_border(
     buf: list[str], width: int, height: int, fps: float = 0.0, frame_time: float = 0.0
 ) -> list[str]:
-    """Render a border around the buffer.
+    """Render a traditional border around the buffer."""
-
-    Args:
-        buf: Input buffer (list of strings)
-        width: Display width in characters
-        height: Display height in rows
-        fps: Current FPS to display in top border (optional)
-        frame_time: Frame time in ms to display in bottom border (optional)
-
-    Returns:
-        Buffer with border applied
-    """
     if not buf or width < 3 or height < 3:
         return buf
 
     inner_w = width - 2
     inner_h = height - 2
 
-    # Crop buffer to fit inside border
     cropped = []
     for i in range(min(inner_h, len(buf))):
         line = buf[i]
-        # Calculate visible width (excluding ANSI codes)
         visible_len = len(_strip_ansi(line))
         if visible_len > inner_w:
-            # Truncate carefully - this is approximate for ANSI text
             cropped.append(line[:inner_w])
         else:
             cropped.append(line + " " * (inner_w - visible_len))
 
-    # Pad with empty lines if needed
     while len(cropped) < inner_h:
         cropped.append(" " * inner_w)
 
-    # Build borders
     if fps > 0:
         fps_str = f" FPS:{fps:.0f}"
         if len(fps_str) < inner_w:
@@ -248,10 +169,8 @@ def render_border(
     else:
         bottom_border = "└" + "─" * inner_w + "┘"
 
-    # Build result with left/right borders
     result = [top_border]
     for line in cropped:
-        # Ensure exactly inner_w characters before adding right border
         if len(line) < inner_w:
             line = line + " " * (inner_w - len(line))
         elif len(line) > inner_w:
@@ -262,14 +181,107 @@ def render_border(
     return result
 
 
+def render_ui_panel(
+    buf: list[str],
+    width: int,
+    height: int,
+    ui_panel,
+    fps: float = 0.0,
+    frame_time: float = 0.0,
+) -> list[str]:
+    """Render buffer with a right-side UI panel."""
+    from engine.pipeline.ui import UIPanel
+
+    if not isinstance(ui_panel, UIPanel):
+        return _render_simple_border(buf, width, height, fps, frame_time)
+
+    panel_width = min(ui_panel.config.panel_width, width - 4)
+    main_width = width - panel_width - 1
+
+    panel_lines = ui_panel.render(panel_width, height)
+
+    main_buf = buf[: height - 2]
+    main_result = _render_simple_border(
+        main_buf, main_width + 2, height, fps, frame_time
+    )
+
+    combined = []
+    for i in range(height):
+        if i < len(main_result):
+            main_line = main_result[i]
+            if len(main_line) >= 2:
+                main_content = (
+                    main_line[1:-1] if main_line[-1] in "│┌┐└┘" else main_line[1:]
+                )
+                main_content = main_content.ljust(main_width)[:main_width]
+            else:
+                main_content = " " * main_width
+        else:
+            main_content = " " * main_width
+
+        panel_idx = i
+        panel_line = (
+            panel_lines[panel_idx][:panel_width].ljust(panel_width)
+            if panel_idx < len(panel_lines)
+            else " " * panel_width
+        )
+
+        separator = "│" if 0 < i < height - 1 else "┼" if i == 0 else "┴"
+        combined.append(main_content + separator + panel_line)
+
+    return combined
+
+
+def render_border(
+    buf: list[str],
+    width: int,
+    height: int,
+    fps: float = 0.0,
+    frame_time: float = 0.0,
+    border_mode: BorderMode | bool = BorderMode.SIMPLE,
+) -> list[str]:
+    """Render a border or UI panel around the buffer.
+
+    Args:
+        buf: Input buffer
+        width: Display width
+        height: Display height
+        fps: FPS for top border
+        frame_time: Frame time for bottom border
+        border_mode: Border rendering mode
+
+    Returns:
+        Buffer with border/panel applied
+    """
+    # Normalize border_mode to BorderMode enum
+    if isinstance(border_mode, bool):
+        border_mode = BorderMode.SIMPLE if border_mode else BorderMode.OFF
+
+    if border_mode == BorderMode.UI:
+        # UI panel requires a UIPanel instance (injected separately)
+        # For now, this will be called by displays that have a ui_panel attribute
+        # This function signature doesn't include ui_panel, so we'll handle it in render_ui_panel
+        # Fall back to simple border if no panel available
+        return _render_simple_border(buf, width, height, fps, frame_time)
+    elif border_mode == BorderMode.SIMPLE:
+        return _render_simple_border(buf, width, height, fps, frame_time)
+    else:
+        return buf
+
+
 __all__ = [
     "Display",
     "DisplayRegistry",
     "get_monitor",
     "render_border",
+    "render_ui_panel",
+    "BorderMode",
     "TerminalDisplay",
     "NullDisplay",
     "WebSocketDisplay",
-    "SixelDisplay",
     "MultiDisplay",
+    "PygameDisplay",
 ]
+
+if _MODERNGL_AVAILABLE:
+    __all__.append("ModernGLDisplay")

engine/display/backends/kitty.py

@@ -1,180 +0,0 @@
-"""
-Kitty graphics display backend - renders using kitty's native graphics protocol.
-"""
-
-import time
-
-from engine.display.renderer import get_default_font_path, parse_ansi
-
-
-def _encode_kitty_graphic(image_data: bytes, width: int, height: int) -> bytes:
-    """Encode image data using kitty's graphics protocol."""
-    import base64
-
-    encoded = base64.b64encode(image_data).decode("ascii")
-
-    chunks = []
-    for i in range(0, len(encoded), 4096):
-        chunk = encoded[i : i + 4096]
-        if i == 0:
-            chunks.append(f"\x1b_Gf=100,t=d,s={width},v={height},c=1,r=1;{chunk}\x1b\\")
-        else:
-            chunks.append(f"\x1b_Gm={height};{chunk}\x1b\\")
-
-    return "".join(chunks).encode("utf-8")
-
-
-class KittyDisplay:
-    """Kitty graphics display backend using kitty's native protocol."""
-
-    width: int = 80
-    height: int = 24
-
-    def __init__(self, cell_width: int = 9, cell_height: int = 16):
-        self.width = 80
-        self.height = 24
-        self.cell_width = cell_width
-        self.cell_height = cell_height
-        self._initialized = False
-        self._font_path = None
-
-    def init(self, width: int, height: int, reuse: bool = False) -> None:
-        """Initialize display with dimensions.
-
-        Args:
-            width: Terminal width in characters
-            height: Terminal height in rows
-            reuse: Ignored for KittyDisplay (protocol doesn't support reuse)
-        """
-        self.width = width
-        self.height = height
-        self._initialized = True
-
-    def _get_font_path(self) -> str | None:
-        """Get font path from env or detect common locations."""
-        import os
-
-        if self._font_path:
-            return self._font_path
-
-        env_font = os.environ.get("MAINLINE_KITTY_FONT")
-        if env_font and os.path.exists(env_font):
-            self._font_path = env_font
-            return env_font
-
-        font_path = get_default_font_path()
-        if font_path:
-            self._font_path = font_path
-
-        return self._font_path
-
-    def show(self, buffer: list[str], border: bool = False) -> None:
-        import sys
-
-        t0 = time.perf_counter()
-
-        # Get metrics for border display
-        fps = 0.0
-        frame_time = 0.0
-        from engine.display import get_monitor
-
-        monitor = get_monitor()
-        if monitor:
-            stats = monitor.get_stats()
-            avg_ms = stats.get("pipeline", {}).get("avg_ms", 0) if stats else 0
-            frame_count = stats.get("frame_count", 0) if stats else 0
-            if avg_ms and frame_count > 0:
-                fps = 1000.0 / avg_ms
-                frame_time = avg_ms
-
-        # Apply border if requested
-        if border:
-            from engine.display import render_border
-
-            buffer = render_border(buffer, self.width, self.height, fps, frame_time)
-
-        img_width = self.width * self.cell_width
-        img_height = self.height * self.cell_height
-
-        try:
-            from PIL import Image, ImageDraw, ImageFont
-        except ImportError:
-            return
-
-        img = Image.new("RGBA", (img_width, img_height), (0, 0, 0, 255))
-        draw = ImageDraw.Draw(img)
-
-        font_path = self._get_font_path()
-        font = None
-        if font_path:
-            try:
-                font = ImageFont.truetype(font_path, self.cell_height - 2)
-            except Exception:
-                font = None
-
-        if font is None:
-            try:
-                font = ImageFont.load_default()
-            except Exception:
-                font = None
-
-        for row_idx, line in enumerate(buffer[: self.height]):
-            if row_idx >= self.height:
-                break
-
-            tokens = parse_ansi(line)
-            x_pos = 0
-            y_pos = row_idx * self.cell_height
-
-            for text, fg, bg, bold in tokens:
-                if not text:
-                    continue
-
-                if bg != (0, 0, 0):
-                    bbox = draw.textbbox((x_pos, y_pos), text, font=font)
-                    draw.rectangle(bbox, fill=(*bg, 255))
-
-                if bold and font:
-                    draw.text((x_pos - 1, y_pos - 1), text, fill=(*fg, 255), font=font)
-
-                draw.text((x_pos, y_pos), text, fill=(*fg, 255), font=font)
-
-                if font:
-                    x_pos += draw.textlength(text, font=font)
-
-        from io import BytesIO
-
-        output = BytesIO()
-        img.save(output, format="PNG")
-        png_data = output.getvalue()
-
-        graphic = _encode_kitty_graphic(png_data, img_width, img_height)
-
-        sys.stdout.buffer.write(graphic)
-        sys.stdout.flush()
-
-        elapsed_ms = (time.perf_counter() - t0) * 1000
-
-        from engine.display import get_monitor
-
-        monitor = get_monitor()
-        if monitor:
-            chars_in = sum(len(line) for line in buffer)
-            monitor.record_effect("kitty_display", elapsed_ms, chars_in, chars_in)
-
-    def clear(self) -> None:
-        import sys
-
-        sys.stdout.buffer.write(b"\x1b_Ga=d\x1b\\")
-        sys.stdout.flush()
-
-    def cleanup(self) -> None:
-        self.clear()
-
-    def get_dimensions(self) -> tuple[int, int]:
-        """Get current dimensions.
-
-        Returns:
-            (width, height) in character cells
-        """
-        return (self.width, self.height)

engine/display/backends/sixel.py

@@ -1,228 +0,0 @@
-"""
-Sixel graphics display backend - renders to sixel graphics in terminal.
-"""
-
-import time
-
-from engine.display.renderer import get_default_font_path, parse_ansi
-
-
-def _encode_sixel(image) -> str:
-    """Encode a PIL Image to sixel format (pure Python)."""
-    img = image.convert("RGBA")
-    width, height = img.size
-    pixels = img.load()
-
-    palette = []
-    pixel_palette_idx = {}
-
-    def get_color_idx(r, g, b, a):
-        if a < 128:
-            return -1
-        key = (r // 32, g // 32, b // 32)
-        if key not in pixel_palette_idx:
-            idx = len(palette)
-            if idx < 256:
-                palette.append((r, g, b))
-                pixel_palette_idx[key] = idx
-        return pixel_palette_idx.get(key, 0)
-
-    for y in range(height):
-        for x in range(width):
-            r, g, b, a = pixels[x, y]
-            get_color_idx(r, g, b, a)
-
-    if not palette:
-        return ""
-
-    if len(palette) == 1:
-        palette = [palette[0], (0, 0, 0)]
-
-    sixel_data = []
-    sixel_data.append(
-        f'"{"".join(f"#{i};2;{r};{g};{b}" for i, (r, g, b) in enumerate(palette))}'
-    )
-
-    for x in range(width):
-        col_data = []
-        for y in range(0, height, 6):
-            bits = 0
-            color_idx = -1
-            for dy in range(6):
-                if y + dy < height:
-                    r, g, b, a = pixels[x, y + dy]
-                    if a >= 128:
-                        bits |= 1 << dy
-                        idx = get_color_idx(r, g, b, a)
-                        if color_idx == -1:
-                            color_idx = idx
-                        elif color_idx != idx:
-                            color_idx = -2
-
-            if color_idx >= 0:
-                col_data.append(
-                    chr(63 + color_idx) + chr(63 + bits)
-                    if bits
-                    else chr(63 + color_idx) + "?"
-                )
-            elif color_idx == -2:
-                pass
-
-        if col_data:
-            sixel_data.append("".join(col_data) + "$")
-        else:
-            sixel_data.append("-" if x < width - 1 else "$")
-
-    sixel_data.append("\x1b\\")
-
-    return "\x1bPq" + "".join(sixel_data)
-
-
-class SixelDisplay:
-    """Sixel graphics display backend - renders to sixel graphics in terminal."""
-
-    width: int = 80
-    height: int = 24
-
-    def __init__(self, cell_width: int = 9, cell_height: int = 16):
-        self.width = 80
-        self.height = 24
-        self.cell_width = cell_width
-        self.cell_height = cell_height
-        self._initialized = False
-        self._font_path = None
-
-    def _get_font_path(self) -> str | None:
-        """Get font path from env or detect common locations."""
-        import os
-
-        if self._font_path:
-            return self._font_path
-
-        env_font = os.environ.get("MAINLINE_SIXEL_FONT")
-        if env_font and os.path.exists(env_font):
-            self._font_path = env_font
-            return env_font
-
-        font_path = get_default_font_path()
-        if font_path:
-            self._font_path = font_path
-
-        return self._font_path
-
-    def init(self, width: int, height: int, reuse: bool = False) -> None:
-        """Initialize display with dimensions.
-
-        Args:
-            width: Terminal width in characters
-            height: Terminal height in rows
-            reuse: Ignored for SixelDisplay
-        """
-        self.width = width
-        self.height = height
-        self._initialized = True
-
-    def show(self, buffer: list[str], border: bool = False) -> None:
-        import sys
-
-        t0 = time.perf_counter()
-
-        # Get metrics for border display
-        fps = 0.0
-        frame_time = 0.0
-        from engine.display import get_monitor
-
-        monitor = get_monitor()
-        if monitor:
-            stats = monitor.get_stats()
-            avg_ms = stats.get("pipeline", {}).get("avg_ms", 0) if stats else 0
-            frame_count = stats.get("frame_count", 0) if stats else 0
-            if avg_ms and frame_count > 0:
-                fps = 1000.0 / avg_ms
-                frame_time = avg_ms
-
-        # Apply border if requested
-        if border:
-            from engine.display import render_border
-
-            buffer = render_border(buffer, self.width, self.height, fps, frame_time)
-
-        img_width = self.width * self.cell_width
-        img_height = self.height * self.cell_height
-
-        try:
-            from PIL import Image, ImageDraw, ImageFont
-        except ImportError:
-            return
-
-        img = Image.new("RGBA", (img_width, img_height), (0, 0, 0, 255))
-        draw = ImageDraw.Draw(img)
-
-        font_path = self._get_font_path()
-        font = None
-        if font_path:
-            try:
-                font = ImageFont.truetype(font_path, self.cell_height - 2)
-            except Exception:
-                font = None
-
-        if font is None:
-            try:
-                font = ImageFont.load_default()
-            except Exception:
-                font = None
-
-        for row_idx, line in enumerate(buffer[: self.height]):
-            if row_idx >= self.height:
-                break
-
-            tokens = parse_ansi(line)
-            x_pos = 0
-            y_pos = row_idx * self.cell_height
-
-            for text, fg, bg, bold in tokens:
-                if not text:
-                    continue
-
-                if bg != (0, 0, 0):
-                    bbox = draw.textbbox((x_pos, y_pos), text, font=font)
-                    draw.rectangle(bbox, fill=(*bg, 255))
-
-                if bold and font:
-                    draw.text((x_pos - 1, y_pos - 1), text, fill=(*fg, 255), font=font)
-
-                draw.text((x_pos, y_pos), text, fill=(*fg, 255), font=font)
-
-                if font:
-                    x_pos += draw.textlength(text, font=font)
-
-        sixel = _encode_sixel(img)
-
-        sys.stdout.buffer.write(sixel.encode("utf-8"))
-        sys.stdout.flush()
-
-        elapsed_ms = (time.perf_counter() - t0) * 1000
-
-        from engine.display import get_monitor
-
-        monitor = get_monitor()
-        if monitor:
-            chars_in = sum(len(line) for line in buffer)
-            monitor.record_effect("sixel_display", elapsed_ms, chars_in, chars_in)
-
-    def clear(self) -> None:
-        import sys
-
-        sys.stdout.buffer.write(b"\x1b[2J\x1b[H")
-        sys.stdout.flush()
-
-    def cleanup(self) -> None:
-        pass
-
-    def get_dimensions(self) -> tuple[int, int]:
-        """Get current dimensions.
-
-        Returns:
-            (width, height) in character cells
-        """
-        return (self.width, self.height)

engine/display/backends/websocket.py

@@ -1,5 +1,14 @@
 """
 WebSocket display backend - broadcasts frame buffer to connected web clients.
+
+TODO: Transform to a true streaming backend with:
+- Proper WebSocket message streaming (currently sends full buffer each frame)
+- Connection pooling and backpressure handling
+- Binary protocol for efficiency (instead of JSON)
+- Client management with proper async handling
+- Mark for deprecation if replaced by a new streaming implementation
+
+Current implementation: Simple broadcast of text frames to all connected clients.
 """
 
 import asyncio

tests/test_display.py

@@ -77,11 +77,13 @@ class TestDisplayRegistry:
         DisplayRegistry.initialize()
         assert DisplayRegistry.get("terminal") == TerminalDisplay
         assert DisplayRegistry.get("null") == NullDisplay
-        from engine.display.backends.sixel import SixelDisplay
+        from engine.display.backends.pygame import PygameDisplay
         from engine.display.backends.websocket import WebSocketDisplay
 
         assert DisplayRegistry.get("websocket") == WebSocketDisplay
-        assert DisplayRegistry.get("sixel") == SixelDisplay
+        assert DisplayRegistry.get("pygame") == PygameDisplay
+        # Removed backends (sixel, kitty) should not be present
+        assert DisplayRegistry.get("sixel") is None
 
     def test_initialize_idempotent(self):
         """initialize can be called multiple times safely."""

tests/test_sixel.py

@@ -1,128 +0,0 @@
-"""
-Tests for engine.display.backends.sixel module.
-"""
-
-from unittest.mock import MagicMock, patch
-
-
-class TestSixelDisplay:
-    """Tests for SixelDisplay class."""
-
-    def test_init_stores_dimensions(self):
-        """init stores dimensions."""
-        from engine.display.backends.sixel import SixelDisplay
-
-        display = SixelDisplay()
-        display.init(80, 24)
-        assert display.width == 80
-        assert display.height == 24
-
-    def test_init_custom_cell_size(self):
-        """init accepts custom cell size."""
-        from engine.display.backends.sixel import SixelDisplay
-
-        display = SixelDisplay(cell_width=12, cell_height=18)
-        assert display.cell_width == 12
-        assert display.cell_height == 18
-
-    def test_show_handles_empty_buffer(self):
-        """show handles empty buffer gracefully."""
-        from engine.display.backends.sixel import SixelDisplay
-
-        display = SixelDisplay()
-        display.init(80, 24)
-
-        with patch("engine.display.backends.sixel._encode_sixel") as mock_encode:
-            mock_encode.return_value = ""
-            display.show([])
-
-    def test_show_handles_pil_import_error(self):
-        """show gracefully handles missing PIL."""
-        from engine.display.backends.sixel import SixelDisplay
-
-        display = SixelDisplay()
-        display.init(80, 24)
-
-        with patch.dict("sys.modules", {"PIL": None}):
-            display.show(["test line"])
-
-    def test_clear_sends_escape_sequence(self):
-        """clear sends clear screen escape sequence."""
-        from engine.display.backends.sixel import SixelDisplay
-
-        display = SixelDisplay()
-
-        with patch("sys.stdout") as mock_stdout:
-            display.clear()
-            mock_stdout.buffer.write.assert_called()
-
-    def test_cleanup_does_nothing(self):
-        """cleanup does nothing."""
-        from engine.display.backends.sixel import SixelDisplay
-
-        display = SixelDisplay()
-        display.cleanup()
-
-
-class TestSixelAnsiParsing:
-    """Tests for ANSI parsing in SixelDisplay."""
-
-    def test_parse_empty_string(self):
-        """handles empty string."""
-        from engine.display.renderer import parse_ansi
-
-        result = parse_ansi("")
-        assert len(result) > 0
-
-    def test_parse_plain_text(self):
-        """parses plain text without ANSI codes."""
-        from engine.display.renderer import parse_ansi
-
-        result = parse_ansi("hello world")
-        assert len(result) == 1
-        text, fg, bg, bold = result[0]
-        assert text == "hello world"
-
-    def test_parse_with_color_codes(self):
-        """parses ANSI color codes."""
-        from engine.display.renderer import parse_ansi
-
-        result = parse_ansi("\033[31mred\033[0m")
-        assert len(result) == 1
-        assert result[0][0] == "red"
-        assert result[0][1] == (205, 49, 49)
-
-    def test_parse_with_bold(self):
-        """parses bold codes."""
-        from engine.display.renderer import parse_ansi
-
-        result = parse_ansi("\033[1mbold\033[0m")
-        assert len(result) == 1
-        assert result[0][0] == "bold"
-        assert result[0][3] is True
-
-    def test_parse_256_color(self):
-        """parses 256 color codes."""
-        from engine.display.renderer import parse_ansi
-
-        result = parse_ansi("\033[38;5;196mred\033[0m")
-        assert len(result) == 1
-        assert result[0][0] == "red"
-
-
-class TestSixelEncoding:
-    """Tests for Sixel encoding."""
-
-    def test_encode_empty_image(self):
-        """handles empty image."""
-        from engine.display.backends.sixel import _encode_sixel
-
-        with patch("PIL.Image.Image") as mock_image:
-            mock_img_instance = MagicMock()
-            mock_img_instance.convert.return_value = mock_img_instance
-            mock_img_instance.size = (0, 0)
-            mock_img_instance.load.return_value = {}
-            mock_image.return_value = mock_img_instance
-
-            result = _encode_sixel(mock_img_instance)
-            assert result == ""