maybe better menu situation

src/import/RomImporter.lua

@@ -4266,6 +4266,7 @@ function RomImporter:_toggleSafeMode()
   self.mods = nil
   self.findInstalled = nil
   self._modSortCache = nil
+  self._cartCaptureCache = nil
   self._modInfoFetch = nil
   self.modNotice = nil
 end
@@ -4592,7 +4593,7 @@ function RomImporter:_selectCart(version, id)
   self.activeCart[version] = id
   self._cartPlan = nil
   -- the MODS panel answers for the cart now, so its cached rows are stale
-  self.mods, self._modSortCache = nil, nil
+  self.mods, self._modSortCache, self._cartCaptureCache = nil, nil, nil
   local scope = self:slotScope(version)
   self.slots[scope] = nil
   self.slotScroll[scope] = nil
@@ -4891,7 +4892,11 @@ end
 -- pins the off ones too, so they are part of the cart it would write.
 function RomImporter:_cartCaptureCount(version)
   if not GameVersion.VERSIONS[version] then return 0 end
-  return #cartModRows(self, version)
+  local cache = self._cartCaptureCache
+  if cache and cache.version == version then return cache.n end
+  local n = #cartModRows(self, version)
+  self._cartCaptureCache = { version = version, n = n }
+  return n
 end
 
 function RomImporter:_cartAuthor()
@@ -5250,6 +5255,7 @@ function RomImporter:_refreshMods()
   local SaveData = require("src.core.SaveData")
   self._cartPlan = nil
   self._profileCache = nil
+  self._cartCaptureCache = nil
   self.findInstalled = nil
   self.safeMode = SaveData.isSafeMode(SaveData.loadOptions())
   -- Once per session, ahead of the first listing: pull in any mod the player
@@ -5284,6 +5290,7 @@ function RomImporter:_refreshMods()
       if m.targetsHere ~= false then kept[#kept + 1] = m end
     end
     self.mods = kept
+    self._cartCaptureCache = { version = self.modScope, n = #kept }
   end
   -- a pin is judged against the whole listing: the cart named it, so it is
   -- listed even where the game filter above would have dropped it

tests/engine/launcher_navigation_perf.lua

@@ -85,4 +85,31 @@ do
     "MOD INDEX panel does not force the full MODS list")
 end
 
+do
+  local calls = 0
+  local old = LauncherMods.list
+  LauncherMods.list = function()
+    calls = calls + 1
+    return {
+      { id = "scoped", name = "Scoped", targetsHere = true },
+      { id = "other", name = "Other", targetsHere = false },
+    }
+  end
+  local imp = setmetatable({
+    modScope = "red",
+    modStraysChecked = true,
+    activeCart = {},
+  }, RomImporter)
+  imp:_refreshMods()
+  local listed = calls
+  check(listed >= 1, "refresh lists installed mods")
+  eq(imp:_cartCaptureCount("red"), 1,
+    "capture count is the targeting subset")
+  eq(calls, listed,
+    "Save as cart does not re-list after the panel already has the rows")
+  eq(imp:_cartCaptureCount("red"), 1, "a second count is the same answer")
+  eq(calls, listed, "and still does not re-list")
+  LauncherMods.list = old
+end
+
 T.finish("launcher_navigation_perf")