Fix scroll tracking during falls in climber

ct.lua

@@ -0,0 +1,30 @@
+local outPath = "C:/src/dotnes"
+local f = io.open(outPath .. "/climb_test.txt", "w")
+
+emu.addEventCallback(function()
+  local fc = emu.getState()["ppu.frameCount"]
+
+  -- Climb up aggressively: alternate UP and A to find ladders and jump
+  if fc > 120 then
+    if fc % 30 < 15 then
+      emu.setInput(0, {up = true})
+    else
+      emu.setInput(0, {a = true, right = true})
+    end
+  end
+
+  -- Log state every 300 frames
+  if fc % 300 == 0 and fc > 0 then
+    local sp_lo = emu.read(0x22, emu.memType.nesInternalRam)
+    local sp_hi = emu.read(0x23, emu.memType.nesInternalRam)
+    local floor = emu.read(0x03C9, emu.memType.nesInternalRam)
+    local state = emu.read(0x03D1, emu.memType.nesInternalRam)
+    local pc = emu.getState()["cpu.pc"]
+    f:write(string.format("Frame %5d: floor=%d state=%d sp=$%02X%02X pc=$%04X\n", fc, floor, state, sp_hi, sp_lo, pc))
+  end
+
+  if fc >= 18000 then
+    f:close()
+    emu.stop(0)
+  end
+end, emu.eventType.endFrame)

ct_out.txt

@@ -0,0 +1,13 @@
+------------------------------------------------------
+Loading rom: climber.nes
+File CRC32: 0xEDEF0A85
+------------------------------------------------------
+[DB] Initialized - 10655 games in DB
+PRG CRC32: 0xD4C12182
+PRG+CHR CRC32: 0x13322F19
+[iNes] Mapper: 0 Sub: 0
+[iNes] PRG ROM: 32 KB
+[iNes] CHR ROM: 8 KB
+[iNes] Mirroring: Horizontal
+[iNes] Battery: No
+[DB] Game not found in database

samples/climber/Program.cs

@@ -385,9 +385,29 @@ static void setup_graphics()
             byte rel_lo = (byte)(actor_yy_lo[ai] - scroll_yy_lo);
             byte rel_hi = (byte)(actor_yy_hi[ai] - scroll_yy_hi);
             if (actor_yy_lo[ai] < scroll_yy_lo) rel_hi = (byte)(rel_hi - 1);
-            if (rel_hi != 0) { actor_onscreen[ai] = 0; continue; }
-            byte screen_y = (byte)(SCREEN_Y_BOTTOM - rel_lo);
-            if (screen_y > 224) { actor_onscreen[ai] = 0; continue; }
+            // For player (ai==0): always compute screen_y and render — never cull.
+            // The original uses signed 16-bit screen_y which never overflows.
+            // For enemies: skip if off-screen.
+            byte screen_y;
+            if (ai == 0)
+            {
+                if (rel_hi == 0)
+                {
+                    screen_y = (byte)(SCREEN_Y_BOTTOM - rel_lo);
+                }
+                else
+                {
+                    // Player off-screen: clamp sprite to bottom edge
+                    screen_y = 224;
+                }
+                player_screen_y = screen_y;
+            }
+            else
+            {
+                if (rel_hi != 0) { actor_onscreen[ai] = 0; continue; }
+                screen_y = (byte)(SCREEN_Y_BOTTOM - rel_lo);
+                if (screen_y > 224) { actor_onscreen[ai] = 0; continue; }
+            }
 
             byte dir = actor_dir[ai];
             byte st = actor_state[ai];
@@ -449,7 +469,6 @@ static void setup_graphics()
             if (st == PACING)
                 oam_meta_spr_pal(actor_x[ai], screen_y, actor_pal[ai], personToSave);
             actor_onscreen[ai] = 1;
-            if (ai == 0) player_screen_y = screen_y;
         }
         // Scoreboard
         oam_off = oam_spr(24, 24, (byte)(0x30 + (score >> 4)), 2, oam_off);