fix(pygame): add fallback border rendering for fonts without box-drawing chars

- Detect if font lacks box-drawing glyphs by testing rendering
- Use pygame.graphics to draw border when text glyphs unavailable
- Adjust content offset to avoid overlapping border
- Ensures border always visible regardless of font support

This improves compatibility across platforms and font configurations.

engine/display/backends/pygame.py

@@ -136,6 +136,21 @@ class PygameDisplay:
         else:
             self._font = pygame.font.SysFont("monospace", self.cell_height - 2)
 
+        # Check if font supports box-drawing characters; if not, try to find one
+        self._use_fallback_border = False
+        if self._font:
+            try:
+                # Test rendering some key box-drawing characters
+                test_chars = ["┌", "─", "┐", "│", "└", "┘"]
+                for ch in test_chars:
+                    surf = self._font.render(ch, True, (255, 255, 255))
+                    # If surface is empty (width=0 or all black), font lacks glyph
+                    if surf.get_width() == 0:
+                        raise ValueError("Missing glyph")
+            except Exception:
+                # Font doesn't support box-drawing, will use line drawing fallback
+                self._use_fallback_border = True
+
         self._initialized = True
 
     def show(self, buffer: list[str], border: bool = False) -> None:
@@ -184,14 +199,26 @@ class PygameDisplay:
                 fps = 1000.0 / avg_ms
                 frame_time = avg_ms
 
-        # Apply border if requested
+        self._screen.fill((0, 0, 0))
+
+        # If border requested but font lacks box-drawing glyphs, use graphical fallback
+        if border and self._use_fallback_border:
+            self._draw_fallback_border(fps, frame_time)
+            # Adjust content area to fit inside border
+            content_offset_x = self.cell_width
+            content_offset_y = self.cell_height
+            self.window_width - 2 * self.cell_width
+            self.window_height - 2 * self.cell_height
+        else:
+            # Normal rendering (with or without text border)
+            content_offset_x = 0
+            content_offset_y = 0
+
             if border:
                 from engine.display import render_border
 
                 buffer = render_border(buffer, self.width, self.height, fps, frame_time)
 
-        self._screen.fill((0, 0, 0))
-
         blit_list = []
 
         for row_idx, line in enumerate(buffer[: self.height]):
@@ -199,7 +226,7 @@ class PygameDisplay:
                 break
 
             tokens = parse_ansi(line)
-            x_pos = 0
+            x_pos = content_offset_x
 
             for text, fg, bg, _bold in tokens:
                 if not text:
@@ -219,10 +246,17 @@ class PygameDisplay:
                         self._glyph_cache[cache_key] = self._font.render(text, True, fg)
 
                 surface = self._glyph_cache[cache_key]
-                blit_list.append((surface, (x_pos, row_idx * self.cell_height)))
+                blit_list.append(
+                    (surface, (x_pos, content_offset_y + row_idx * self.cell_height))
+                )
                 x_pos += self._font.size(text)[0]
 
         self._screen.blits(blit_list)
+
+        # Draw fallback border using graphics if needed
+        if border and self._use_fallback_border:
+            self._draw_fallback_border(fps, frame_time)
+
         self._pygame.display.flip()
 
         elapsed_ms = (time.perf_counter() - t0) * 1000
@@ -231,6 +265,56 @@ class PygameDisplay:
             chars_in = sum(len(line) for line in buffer)
             monitor.record_effect("pygame_display", elapsed_ms, chars_in, chars_in)
 
+    def _draw_fallback_border(self, fps: float, frame_time: float) -> None:
+        """Draw border using pygame graphics primitives instead of text."""
+        if not self._screen or not self._pygame:
+            return
+
+        # Colors
+        border_color = (0, 255, 0)  # Green (like terminal border)
+        text_color = (255, 255, 255)
+
+        # Calculate dimensions
+        x1 = 0
+        y1 = 0
+        x2 = self.window_width - 1
+        y2 = self.window_height - 1
+
+        # Draw outer rectangle
+        self._pygame.draw.rect(
+            self._screen, border_color, (x1, y1, x2 - x1 + 1, y2 - y1 + 1), 1
+        )
+
+        # Draw top border with FPS
+        if fps > 0:
+            fps_text = f" FPS:{fps:.0f}"
+        else:
+            fps_text = ""
+        # We need to render this text with a fallback font that has basic ASCII
+        # Use system font which should have these characters
+        try:
+            font = self._font  # May not have box chars but should have alphanumeric
+            text_surf = font.render(fps_text, True, text_color, (0, 0, 0))
+            text_rect = text_surf.get_rect()
+            # Position on top border, right-aligned
+            text_x = x2 - text_rect.width - 5
+            text_y = y1 + 2
+            self._screen.blit(text_surf, (text_x, text_y))
+        except Exception:
+            pass
+
+        # Draw bottom border with frame time
+        if frame_time > 0:
+            ft_text = f" {frame_time:.1f}ms"
+            try:
+                ft_surf = self._font.render(ft_text, True, text_color, (0, 0, 0))
+                ft_rect = ft_surf.get_rect()
+                ft_x = x2 - ft_rect.width - 5
+                ft_y = y2 - ft_rect.height - 2
+                self._screen.blit(ft_surf, (ft_x, ft_y))
+            except Exception:
+                pass
+
     def clear(self) -> None:
         if self._screen and self._pygame:
             self._screen.fill((0, 0, 0))