From 8f8d5aa1e2182c9bcbaf2f2d45ec29ee4f4869d8 Mon Sep 17 00:00:00 2001
From: Claude <noreply@anthropic.com>
Date: Thu, 25 Jun 2026 17:35:24 +0000
Subject: [PATCH] Fix title Pikachu mirror and post-title window artifact

Two visual bugs both rooted in the 20x18 -> 40x25 viewport widening
(SCREEN_TILES_W 20->40, SCREEN_TILES_H 18->25), which silently broke two
spots that still assumed the GB's 32-wide tilemap / 20-wide screen.

Bug 1 (title Pikachu split/mirrored): the non-overworld render path read
the 40-wide wTileMap through the 32-wide GB VRAM tilemap. render_bg's
.decode_vram wrapped columns at 32 (`and edx,31`), duplicating Pikachu's
left columns onto the far right. Rewrite .decode_vram to read wTileMap
directly with row/col bounds (blank $7F past col 40 / row 25), mirroring
the overworld path's native-width read of wSurroundingTiles. This removes
the lossy VRAM hop and the column wrap entirely, and also lets menus/text
wider than 32 tiles render correctly. The window layer (TILEMAP1) is
untouched.

Bug 2 (post-title blank rectangle): LCDC default $E3 leaves window-enable
(bit 5) permanently set, so render_window runs every frame gated only on
WY/WX. The title's go_to_main_menu leaves H_WY=0, and ResetMapVariables
never reset the window registers, so a stale window block painted over the
overworld. Park the window off-screen (WY=RENDER_H, WX=7) in
ResetMapVariables; dialog code re-shows the box at WY=152/WX=87 when needed.

Both verified to assemble clean (nasm -f coff). Neither bug reproduces
under SKIP_TITLE=1, consistent with both being title-path issues.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GcHpAsg8pwsVATD6LWPsoR
---
 dos_port/src/engine/overworld/overworld.asm |  8 ++++++
 dos_port/src/ppu/ppu.asm                    | 30 +++++++++++++--------
 2 files changed, 27 insertions(+), 11 deletions(-)

diff --git a/dos_port/src/engine/overworld/overworld.asm b/dos_port/src/engine/overworld/overworld.asm
index b81bab8c..73c62629 100644
--- a/dos_port/src/engine/overworld/overworld.asm
+++ b/dos_port/src/engine/overworld/overworld.asm
@@ -884,6 +884,14 @@ ResetMapVariables:
     mov byte [ebp + W_UNUSED_CUR_MAP_TILESET_COPY], al
     mov byte [ebp + W_SPRITE_SET_ID],             al
     mov byte [ebp + W_WALK_BIKE_SURF_STATE_COPY], al
+    ; Park the window layer off-screen on map entry. LCDC bit 5 (window enable)
+    ; is permanently set (LCDC default $E3), so render_window runs every frame
+    ; gated only on WY/WX. The title's go_to_main_menu leaves H_WY=0, which would
+    ; otherwise leak in here and paint a stale window block over the overworld.
+    ; RENDER_H = off-screen; dialog code re-shows the box at WY=152/WX=87.
+    mov byte [ebp + H_WY],  RENDER_H
+    mov byte [ebp + IO_WY], RENDER_H
+    mov byte [ebp + IO_WX], 7
     ret
 
 ; ---------------------------------------------------------------------------
diff --git a/dos_port/src/ppu/ppu.asm b/dos_port/src/ppu/ppu.asm
index 45045b94..7e3f0739 100644
--- a/dos_port/src/ppu/ppu.asm
+++ b/dos_port/src/ppu/ppu.asm
@@ -155,20 +155,28 @@ render_bg:
     jmp .got_tile
 
 .decode_vram:
+    ; Non-overworld path (title / menus / text). Read the 40-wide wTileMap WRAM
+    ; buffer DIRECTLY into bg_surface, mirroring the overworld branch's
+    ; native-width read of wSurroundingTiles above. The old code funnelled the
+    ; 40-wide buffer through the 32-wide GB VRAM tilemap (do_bg_transfer) and
+    ; re-read it with `and edx,31`, which wrapped surface cols 32-47 back to
+    ; cols 0-15 — duplicating the left of the screen onto the right (the title
+    ; Pikachu "mirror"). wTileMap is 40 (SCREEN_TILES_W) x 25 (SCREEN_TILES_H);
+    ; the 48x36 surface's extra cols (>=40) and rows (>=25) render as blank $7F.
     mov eax, ebx
     mov ecx, 48
     xor edx, edx
-    div ecx
-    and edx, 31
-    shl eax, 5
-    add eax, edx
-    mov ecx, GB_TILEMAP0
-    test byte [ebp + IO_LCDC], 1 << 3
-    jz .read_vram
-    mov ecx, GB_TILEMAP1
-.read_vram:
-    add eax, ecx
-    mov al, [ebp + eax]
+    div ecx                       ; EAX = surface row, EDX = surface col
+    cmp eax, SCREEN_TILES_H        ; row >= 25 -> off-buffer
+    jae .vram_blank
+    cmp edx, SCREEN_TILES_W        ; col >= 40 -> right padding
+    jae .vram_blank
+    imul eax, eax, SCREEN_TILES_W  ; row * 40
+    add eax, edx                   ; + col
+    mov al, [ebp + W_TILEMAP + eax]
+    jmp .got_tile
+.vram_blank:
+    mov al, 0x7F                   ; blank space tile (matches ClearScreen fill)
 
 .got_tile: