diff --git a/conf.lua b/conf.lua
index 56a89919..ea6e8384 100644
--- a/conf.lua
+++ b/conf.lua
@@ -60,11 +60,13 @@ function love.conf(t)
   local mobile = osName == "Android" or osName == "iOS"
   local nx = osName == "NX"
   if nx then
-    -- Switch (love-nx): docked/handheld 720p surface; no desktop resize hints.
+    -- Switch (love-nx): hint handheld 720p. SDL auto-switches portable↔dock
+    -- (720p↔1080p) only when the window is resizable and not exclusive
+    -- fullscreen; NxDisplay.sync also applies the size on boot and dock change.
     t.window.width = 1280
     t.window.height = 720
-    t.window.fullscreen = true
-    t.window.resizable = false
+    t.window.fullscreen = false
+    t.window.resizable = true
     t.window.highdpi = false
   elseif mobile then
     -- resizable is what unlocks orientation.  SDL's Android backend, given no
diff --git a/docs/switch-development.md b/docs/switch-development.md
index e16db6b9..87272d6a 100644
--- a/docs/switch-development.md
+++ b/docs/switch-development.md
@@ -49,10 +49,11 @@ the transfer runbook).
 - Hardware evidence for Phase 0 probe, ROM import, naming A/B, save/suspend, fused NRO — see `docs/switch-hardware-evidence.md`
 - Path-gated CI selftest + canonical fused PR artifact; release Switch hard-fail
 - Save editor pad/touch input (virtual cursor, A click, B close) — see `tools/save-editor/README.md`
+- Dynamic display size on NX only: handheld **1280×720**, docked/TV **1920×1080** (`src/core/NxDisplay.lua` + resizable conf so love-nx SDL can follow dock/undock at runtime)
 
 ### Known gaps / welcome contributions
 
-- Docked vs handheld soak, long-play soak (≥30 min)
+- Docked vs handheld soak (≥30 min) and Lite coverage — resolution switch is implemented; long soak still welcome
 - Switch Lite and fuller Pro Controller / third-party pad matrices
 - Applet Mode remains unsupported by design (title override required)
 - `nxlink` / netloader contrib fast-loop (deferred — see [switch-transfer.md](switch-transfer.md))
@@ -476,7 +477,7 @@ Operator evidence lives in `docs/switch-hardware-evidence.md`. **Do not invent p
 | P0-12 | Fused NRO boots without adjacent `game.love` | **pass** | T24 — `docs/switch-hardware-evidence.md` |
 | P0-14 | Fused NRO MTP round-trip SHA-256 | **pass** | T24 — first artifact `b019e2e8…` @ `6fb5602` (redeploy after Blue fix) |
 | P0-15 | Replace NRO only; saves persist | **pass** | T24 — operator NRO-only update keeps saves |
-| P1-01 | Docked vs handheld spot-check | **deferred** | Not exercised on OLED dock yet |
+| P1-01 | Docked vs handheld spot-check | **deferred** | Code: `NxDisplay` 720p↔1080p; OLED dock soak not recorded yet |
 | P1-02 | Applet Mode documented unsupported | **pass** | Title override required; Album path not validated |
 | P1-03 | Long-play soak (≥30 min) | **deferred** | No soak session recorded |
 | P1-04 | Reboot persistence | **pass** | T19 |
diff --git a/main.lua b/main.lua
index ca4c0751..ad7d7183 100644
--- a/main.lua
+++ b/main.lua
@@ -11,6 +11,7 @@
 local editorMode = os.getenv("POKEPORT_EDITOR") == "1" or POKEPORT_EDITOR_MODE == true
 
 local SwitchDiagnostics = require("src.debug.SwitchDiagnostics")
+local NxDisplay = require("src.core.NxDisplay")
 
 -- Lua errors: persist a redacted trace in the save dir and surface a hint.
 do
@@ -233,6 +234,9 @@ function love.load(args)
     end
   end
   love.graphics.setDefaultFilter("nearest", "nearest")
+  -- NX: handheld 720p / docked 1080p. Runs for every boot path (launcher,
+  -- editor, scripted); no-op on desktop/mobile.
+  NxDisplay.sync()
 
   -- Standalone editor.  A bare `--editor` run has no launcher behind it, so
   -- Close quits; --save points it at a specific file, otherwise it opens the
@@ -299,6 +303,8 @@ end
 
 function love.update(dt)
   SwitchDiagnostics.maybeFlush(false)
+  -- NX only (no-op elsewhere): follow dock/undock without waiting for SDL.
+  NxDisplay.sync()
   if editorMode then return EditorApp.update(dt) end
   if TouchEditor then return TouchEditor.update(dt) end
   if Importer then return Importer:update(dt) end
diff --git a/src/core/NxDisplay.lua b/src/core/NxDisplay.lua
new file mode 100644
index 00000000..65b815b6
--- /dev/null
+++ b/src/core/NxDisplay.lua
@@ -0,0 +1,96 @@
+-- Switch-only display size: handheld 1280x720, docked (TV) 1920x1080.
+-- love-nx's SDL backend can auto-resize on dock/undock when the window is
+-- resizable; this module also syncs on boot and every frame so a docked
+-- launch is not stuck at the conf.lua 720p hint until the next mode change.
+
+local Platform = require("src.core.Platform")
+
+local NxDisplay = {}
+
+NxDisplay.HANDHELD_W, NxDisplay.HANDHELD_H = 1280, 720
+NxDisplay.DOCKED_W, NxDisplay.DOCKED_H = 1920, 1080
+
+-- AppletOperationMode from libnx: Handheld = 0, Console (docked) = 1.
+local MODE_HANDHELD = 0
+local MODE_CONSOLE = 1
+
+-- Test hooks (nil = use live Platform / FFI / love.window).
+NxDisplay._forceNXForTests = nil
+NxDisplay._operationModeForTests = nil
+
+local ffiOk, ffiC
+
+local function ensureFfi()
+  if ffiOk ~= nil then return ffiOk end
+  ffiOk = false
+  local ok, ffi = pcall(require, "ffi")
+  if not ok or not ffi then return false end
+  -- Redefinition is fine across hot reload / tests; we only need the symbol.
+  pcall(ffi.cdef, [[
+    unsigned char appletGetOperationMode(void);
+  ]])
+  local probeOk = pcall(function()
+    return ffi.C.appletGetOperationMode
+  end)
+  if not probeOk then return false end
+  ffiC = ffi.C
+  ffiOk = true
+  return true
+end
+
+local function isNX()
+  if NxDisplay._forceNXForTests ~= nil then
+    return not not NxDisplay._forceNXForTests
+  end
+  return Platform.isNX()
+end
+
+-- Returns AppletOperationMode or nil when unavailable.
+function NxDisplay.operationMode()
+  if NxDisplay._operationModeForTests ~= nil then
+    return NxDisplay._operationModeForTests
+  end
+  if not ensureFfi() or not ffiC then return nil end
+  local ok, mode = pcall(function()
+    return tonumber(ffiC.appletGetOperationMode())
+  end)
+  if not ok then return nil end
+  return mode
+end
+
+-- Map operation mode → framebuffer size. Unknown / nil → handheld 720p.
+function NxDisplay.desiredSize(mode)
+  if mode == nil then mode = NxDisplay.operationMode() end
+  if mode == MODE_CONSOLE then
+    return NxDisplay.DOCKED_W, NxDisplay.DOCKED_H
+  end
+  return NxDisplay.HANDHELD_W, NxDisplay.HANDHELD_H
+end
+
+-- Apply handheld/dock size when on NX and the window differs. No-op elsewhere.
+-- Returns true when setMode ran.
+function NxDisplay.sync()
+  if not isNX() then return false end
+  if not (love and love.window and love.window.getMode and love.window.setMode) then
+    return false
+  end
+  local wantW, wantH = NxDisplay.desiredSize()
+  local curW, curH, flags = love.window.getMode()
+  flags = flags or {}
+  if curW == wantW and curH == wantH
+     and flags.fullscreen == false and flags.resizable == true then
+    return false
+  end
+  flags.fullscreen = false
+  flags.resizable = true
+  love.window.setMode(wantW, wantH, flags)
+  return true
+end
+
+function NxDisplay._resetForTests()
+  NxDisplay._forceNXForTests = nil
+  NxDisplay._operationModeForTests = nil
+  ffiOk, ffiC = nil, nil
+end
+
+return NxDisplay
diff --git a/tests/engine/nx_display_test.lua b/tests/engine/nx_display_test.lua
new file mode 100644
index 00000000..23d7284f
--- /dev/null
+++ b/tests/engine/nx_display_test.lua
@@ -0,0 +1,123 @@
+-- NX handheld/dock display sync (portable 720p / docked 1080p).
+-- Self-contained: luajit tests/engine/nx_display_test.lua
+
+package.path = "./?.lua;./?/init.lua;" .. package.path
+
+local T = require("tests.harness")
+local check = T.check
+local eq = T.eq
+
+local savedLove = _G.love
+
+package.loaded["src.core.Platform"] = nil
+package.loaded["src.core.NxDisplay"] = nil
+
+local NxDisplay = require("src.core.NxDisplay")
+
+local function withWindow(state, fn)
+  local setCalls = {}
+  _G.love = {
+    system = {
+      getOS = function() return state.os or "NX" end,
+    },
+    window = {
+      getMode = function()
+        return state.w, state.h, {
+          fullscreen = state.fullscreen,
+          resizable = state.resizable,
+          vsync = 1,
+        }
+      end,
+      setMode = function(w, h, flags)
+        setCalls[#setCalls + 1] = { w = w, h = h, flags = flags }
+        state.w, state.h = w, h
+        state.fullscreen = flags and flags.fullscreen
+        state.resizable = flags and flags.resizable
+      end,
+    },
+  }
+  package.loaded["src.core.Platform"] = nil
+  require("src.core.Platform")._resetForTests()
+  NxDisplay._resetForTests()
+  package.loaded["src.core.NxDisplay"] = nil
+  NxDisplay = require("src.core.NxDisplay")
+  local ok, err = pcall(fn, setCalls)
+  _G.love = savedLove
+  package.loaded["src.core.Platform"] = nil
+  package.loaded["src.core.NxDisplay"] = nil
+  NxDisplay = require("src.core.NxDisplay")
+  NxDisplay._resetForTests()
+  if not ok then error(err) end
+end
+
+-- Size mapping
+do
+  local w, h = NxDisplay.desiredSize(0)
+  eq(w, 1280, "handheld mode → 1280 wide")
+  eq(h, 720, "handheld mode → 720 tall")
+  w, h = NxDisplay.desiredSize(1)
+  eq(w, 1920, "console/docked mode → 1920 wide")
+  eq(h, 1080, "console/docked mode → 1080 tall")
+  w, h = NxDisplay.desiredSize(nil)
+  eq(w, 1280, "nil mode falls back to handheld width")
+  eq(h, 720, "nil mode falls back to handheld height")
+  w, h = NxDisplay.desiredSize(99)
+  eq(w, 1280, "unknown mode falls back to handheld width")
+end
+
+-- Non-NX: sync is a no-op
+withWindow({ os = "Linux", w = 1024, h = 768, fullscreen = false, resizable = true }, function(setCalls)
+  NxDisplay._forceNXForTests = false
+  NxDisplay._operationModeForTests = 1
+  eq(NxDisplay.sync(), false, "sync returns false off NX")
+  eq(#setCalls, 0, "sync never calls setMode off NX")
+end)
+
+-- NX handheld already correct: no setMode
+withWindow({
+  os = "NX", w = 1280, h = 720, fullscreen = false, resizable = true,
+}, function(setCalls)
+  NxDisplay._forceNXForTests = true
+  NxDisplay._operationModeForTests = 0
+  eq(NxDisplay.sync(), false, "sync skips setMode when already handheld")
+  eq(#setCalls, 0, "no setMode calls when size+flags match handheld")
+end)
+
+-- NX docked boot from 720p hint → 1080p
+withWindow({
+  os = "NX", w = 1280, h = 720, fullscreen = true, resizable = false,
+}, function(setCalls)
+  NxDisplay._forceNXForTests = true
+  NxDisplay._operationModeForTests = 1
+  eq(NxDisplay.sync(), true, "sync upgrades docked boot to 1080p")
+  eq(#setCalls, 1, "one setMode on docked boot")
+  eq(setCalls[1].w, 1920, "docked setMode width")
+  eq(setCalls[1].h, 1080, "docked setMode height")
+  eq(setCalls[1].flags.fullscreen, false, "docked setMode clears exclusive fullscreen")
+  eq(setCalls[1].flags.resizable, true, "docked setMode enables resizable for SDL backup")
+end)
+
+-- NX undock: 1080p → 720p
+withWindow({
+  os = "NX", w = 1920, h = 1080, fullscreen = false, resizable = true,
+}, function(setCalls)
+  NxDisplay._forceNXForTests = true
+  NxDisplay._operationModeForTests = 0
+  eq(NxDisplay.sync(), true, "sync shrinks to handheld after undock")
+  eq(setCalls[1].w, 1280, "undock setMode width")
+  eq(setCalls[1].h, 720, "undock setMode height")
+end)
+
+-- Flags-only fix when size already matches
+withWindow({
+  os = "NX", w = 1280, h = 720, fullscreen = true, resizable = false,
+}, function(setCalls)
+  NxDisplay._forceNXForTests = true
+  NxDisplay._operationModeForTests = 0
+  eq(NxDisplay.sync(), true, "sync fixes fullscreen/resizable even if size matches")
+  eq(setCalls[1].w, 1280, "flags-only setMode keeps handheld width")
+  eq(setCalls[1].flags.fullscreen, false, "flags-only clears fullscreen")
+  eq(setCalls[1].flags.resizable, true, "flags-only sets resizable")
+end)
+
+T.finish("nx_display")