fix: track Black BG changes in background segments for correct DH backfill

src/teletext/renderer.py

@@ -252,6 +252,9 @@ class TeletextCanvas(QWidget):
         held_char = 0x20 # Space
         double_height = False
         
+        last_visible_idx = -1
+        bg_segments = [(0, bg)] # Track BG changes: (index, color)
+        
         y = row * self.cell_h
         
         data = b''
@@ -311,12 +314,33 @@ class TeletextCanvas(QWidget):
                     graphics_mode = True
                 elif byte_val == 0x1C: # Black BG
                     bg = COLORS[0]
+                    bg_segments.append((c, bg))
                 elif byte_val == 0x1D: # New BG
                     bg = fg
+                    bg_segments.append((c, bg))
+                    
                 elif byte_val == 0x0C: # Normal Height
                     double_height = False
                 elif byte_val == 0x0D: # Double Height
                     double_height = True
+                    # Backfill Height if we are in leading controls
+                    if last_visible_idx == -1:
+                        # Update occlusion mask for 0..c-1
+                        for k in range(c):
+                            next_occlusion_mask[k] = True
+                            
+                        # Repaint 0..c-1 with DH
+                        if draw_bg:
+                            # Repaint each cell with its historical BG
+                            for k in range(c):
+                                # Resolve BG for k
+                                cell_bg = bg_segments[0][1]
+                                for idx, color in reversed(bg_segments):
+                                    if idx <= k:
+                                        cell_bg = color
+                                        break
+                                painter.fillRect(k * self.cell_w, y, self.cell_w, self.cell_h * 2, cell_bg)
+
                 elif byte_val == 0x19: # Contiguous Graphics
                     contiguous = True
                 elif byte_val == 0x1A: # Separated Graphics
@@ -326,6 +350,23 @@ class TeletextCanvas(QWidget):
                 elif byte_val == 0x1F: # Release Graphics
                     hold_graphics = False
             
+            # Update visibility tracker
+            # If it's a control code, it's "invisible" (space) UNLESS Held Graphics draws something
+            visible_content = False
+            if is_control:
+                if hold_graphics and graphics_mode:
+                    visible_content = True
+            else:
+                 # Treat Space (0x20) as "invisible" for backfill purposes
+                 # This allows backfilling height over leading spaces in banners
+                 if byte_val > 0x20:
+                     visible_content = True
+                 
+            if visible_content:
+                # If this is the first visible char, mark it
+                if last_visible_idx == -1:
+                    last_visible_idx = c
+
             # Record Double Height for next row
             if double_height and not is_occluded:
                 next_occlusion_mask[c] = True