Stop offering the route Pokemon Centers as fly destinations

src/world/Map.lua

@@ -24,6 +24,13 @@ local NO_SHORE_TILESETS = { SHIP_PORT = true }
 -- what counts as "outside" for the wLastMap memory (CheckIfInOutsideMap)
 local OUTSIDE_TILESETS = { "OVERWORLD", "PLATEAU" }
 
+-- pokered's fly destination gate: BuildFlyLocationsList
+-- (engine/items/town_map.asm) walks map ids 0..NUM_CITY_MAPS-1, the eleven
+-- towns PALLET_TOWN..SAFFRON_CITY, so routes never appear even though
+-- ROUTE_4/ROUTE_10 carry fly-warp landing spots (those exist for the
+-- dungeon-escape/heal tables, special_warps.asm FlyWarpDataPtr)
+local NUM_CITY_MAPS = 11
+
 -- warp pads and fall-through holes (data/tilesets/warp_pad_hole_tile_ids
 -- .asm WarpPadAndHoleData); a tileset record carrying warpPadTiles
 -- ({ [tileId] = "pad"|"hole" }) wins over these vanilla rows
@@ -153,6 +160,17 @@ function Map.isOutside(def, tilesets)
   return false
 end
 
+-- FLY destination (LoadTownMap_Fly / BuildFlyLocationsList): the eleven
+-- towns, map indices 0..NUM_CITY_MAPS-1.  ROUTE_4 and ROUTE_10 are outdoor
+-- and have fly warps but are not towns, so the outdoor test alone offered
+-- their Pokemon Centers as fly targets (#788).  Maps without a vanilla
+-- index (mod-authored) keep the old outdoor/PLATEAU surface test, which is
+-- how a mod adds its own fly town.
+function Map.isFlyTown(def)
+  if def.index ~= nil then return def.index < NUM_CITY_MAPS end
+  return Map.isOutdoor(def) or def.tileset == "PLATEAU"
+end
+
 -- region groups maps a rule applies to without naming them; the id prefix
 -- is the fallback for caches that predate the property
 function Map.inRegion(def, region, prefix)

src/world/OverworldController.lua

@@ -364,7 +364,12 @@ function OverworldState:setMap(mapId, x, y, facing, opts)
     Game.save.flashLit = nil
     self:setDark(false)
   end
-  if Game.data.field.flyWarps[mapId] then
+  -- MarkTownVisitedAndLoadToggleableObjects marks towns only (cp
+  -- FIRST_ROUTE_MAP): the fly-warp table also carries the ROUTE_4/ROUTE_10
+  -- Pokemon Centers and the dungeon escape spots, and entering those never
+  -- sets a wTownVisitedFlag bit, so it must not set save.visited either (#788)
+  local mapDef = Game.data.maps[mapId]
+  if Game.data.field.flyWarps[mapId] and mapDef and Map.isFlyTown(mapDef) then
     Game.save.visited = Game.save.visited or {}
     Game.save.visited[mapId] = true
   end