diff --git a/main.lua b/main.lua
index 94824275..95d26351 100644
--- a/main.lua
+++ b/main.lua
@@ -1421,6 +1421,7 @@ function love.run()
   if love.timer then love.timer.step() end
 
   local FrameCap = require("src.core.FrameCap")
+  _G.POKEPORT_LOOP_PANEL_SYNC = true
   FrameCap.bootPanelSync()
   local RefreshRate = require("src.core.RefreshRate")
   local FixedStep = require("src.core.FixedStep")
diff --git a/src/core/FrameCap.lua b/src/core/FrameCap.lua
index c5feb033..1082cf85 100644
--- a/src/core/FrameCap.lua
+++ b/src/core/FrameCap.lua
@@ -23,7 +23,12 @@ end
 -- numeric FrameCap default.  Android/iOS/UWP need uncapped probe isolation
 -- so composed GLES swapchains can lock; PortMaster handhelds likewise
 -- follow KMSDRM through PresentSync.
+function FrameCap.loopSupportsPanelSync()
+  return rawget(_G, "POKEPORT_LOOP_PANEL_SYNC") == true
+end
+
 function FrameCap.prefersPanelSync()
+  if not FrameCap.loopSupportsPanelSync() then return false end
   if isHandheldEnv() then return true end
   if love and love.system and love.system.getOS then
     local osName = love.system.getOS()
@@ -56,7 +61,10 @@ FrameCap.current = FrameCap.DEFAULT
 function FrameCap.normalize(value)
   value = tonumber(value)
   if not value then return FrameCap.DEFAULT end
-  if value <= 0 then return FrameCap.DISPLAY end
+  if value <= 0 then
+    if FrameCap.loopSupportsPanelSync() then return FrameCap.DISPLAY end
+    return FrameCap.DEFAULT
+  end
   local best, bestDiff = FrameCap.DEFAULT, math.huge
   for _, step in ipairs(FrameCap.STEPS) do
     local diff = math.abs(step - value)
diff --git a/src/core/Version.lua b/src/core/Version.lua
index bc4f7cc1..ae234d4d 100644
--- a/src/core/Version.lua
+++ b/src/core/Version.lua
@@ -7,7 +7,7 @@ local Version = {
   engine = "0.0.0-dev",   -- game/engine release (semver).  Repo default is the
                           -- "-dev" placeholder; CI stamps the real X.Y.Z into
                           -- the packed game.love only, never the working tree.
-  shell = 1,              -- native-shell contract this build implements
+  shell = 2,              -- native-shell contract this build implements
   payloadHost = "love",   -- native host family for in-place Lua payloads.
                           -- A payload must name the same family; this prevents
                           -- mounting code packaged for a different native host.
diff --git a/tests/engine/frame_cap_display.lua b/tests/engine/frame_cap_display.lua
index 2c201952..84d9d54d 100644
--- a/tests/engine/frame_cap_display.lua
+++ b/tests/engine/frame_cap_display.lua
@@ -1,6 +1,7 @@
 package.path = "./?.lua;./?/init.lua;" .. package.path
 
 local T = require("tests.modkit")
+_G.POKEPORT_LOOP_PANEL_SYNC = true
 local FrameCap = require("src.core.FrameCap")
 local RefreshRate = require("src.core.RefreshRate")
 
diff --git a/tests/engine/frame_cap_legacy_loop_2106.lua b/tests/engine/frame_cap_legacy_loop_2106.lua
new file mode 100644
index 00000000..d06858bb
--- /dev/null
+++ b/tests/engine/frame_cap_legacy_loop_2106.lua
@@ -0,0 +1,44 @@
+package.path = "./?.lua;./?/init.lua;" .. package.path
+
+local T = require("tests.modkit")
+_G.POKEPORT_LOOP_PANEL_SYNC = nil
+package.loaded["src.core.FrameCap"] = nil
+local FrameCap = require("src.core.FrameCap")
+
+T.eq(FrameCap.loopSupportsPanelSync(), false,
+  "no marker means a run loop that predates DISPLAY pacing")
+T.eq(FrameCap.normalize(0), FrameCap.DEFAULT,
+  "a stored DISPLAY cap paces at the default instead of 1/0")
+T.eq(FrameCap.normalize(-5), FrameCap.DEFAULT, "and so does a negative cap")
+
+love = love or {}
+love.system = love.system or {}
+local savedGetOS = love.system.getOS
+local savedGetenv = os.getenv
+os.getenv = function(name)
+  if name == "POKEPORT_HANDHELD" then return "1" end
+  return savedGetenv(name)
+end
+for _, osName in ipairs({ "Android", "iOS", "UWP" }) do
+  love.system.getOS = function() return osName end
+  T.eq(FrameCap.prefersPanelSync(), false,
+    osName .. " does not prefer panel sync under a legacy loop")
+  FrameCap.current = FrameCap.DEFAULT
+  FrameCap.applyOptions({})
+  T.check(FrameCap.current > 0, osName .. " boots with a positive cap")
+  FrameCap.applyOptions({ fpsCap = 0 })
+  T.check(FrameCap.current > 0, osName .. " keeps a positive cap for a stored 0")
+  FrameCap.current = FrameCap.DEFAULT
+  FrameCap.bootPanelSync()
+  T.check(FrameCap.current > 0, "bootPanelSync stays positive on " .. osName)
+end
+os.getenv = savedGetenv
+love.system.getOS = savedGetOS
+
+_G.POKEPORT_LOOP_PANEL_SYNC = true
+T.eq(FrameCap.normalize(0), FrameCap.DISPLAY,
+  "the marker from a current love.run restores DISPLAY")
+_G.POKEPORT_LOOP_PANEL_SYNC = nil
+package.loaded["src.core.FrameCap"] = nil
+
+T.finish("frame cap legacy loop #2106")
diff --git a/tests/engine/gen2_options_menu_translation_test.lua b/tests/engine/gen2_options_menu_translation_test.lua
index 787dbe8e..5d003b5b 100644
--- a/tests/engine/gen2_options_menu_translation_test.lua
+++ b/tests/engine/gen2_options_menu_translation_test.lua
@@ -10,6 +10,7 @@
 -- OPTION screen's equivalent rows), plus a vanilla no-mod case proving the
 -- fallback is unchanged.
 package.path = "./?.lua;./?/init.lua;" .. package.path
+_G.POKEPORT_LOOP_PANEL_SYNC = true
 
 local T = require("tests.harness")
 
diff --git a/tests/engine/present_sync_logic.lua b/tests/engine/present_sync_logic.lua
index adf580ec..79a89754 100644
--- a/tests/engine/present_sync_logic.lua
+++ b/tests/engine/present_sync_logic.lua
@@ -1,6 +1,7 @@
 package.path = "./?.lua;./?/init.lua;" .. package.path
 
 local T = require("tests.modkit")
+_G.POKEPORT_LOOP_PANEL_SYNC = true
 local FrameCap = require("src.core.FrameCap")
 local RefreshRate = require("src.core.RefreshRate")
 local VSync = require("src.core.VSync")