diff --git a/docs/modernization/debt.md b/docs/modernization/debt.md
index d19fcd25..393bb332 100644
--- a/docs/modernization/debt.md
+++ b/docs/modernization/debt.md
@@ -18,6 +18,11 @@ agent or engineer to remove them without reconstructing context from chat.
 
 ## Reductions
 
+- 2026-06-17: `DEBT-0003` was narrowed again.
+  `src/platform_windows/windows_runtime_shell.cpp` now binds a local
+  `MainWindowSession` object for the live Win32 session, and the window handle,
+  title buffer, and sandbox flag now live on that explicit runtime-owned
+  session instead of the retired retained accessor pocket.
 - 2026-06-17: `DEBT-0016`/`DEBT-0017`/`DEBT-0050`/`DEBT-0051`/`DEBT-0053`/
   `DEBT-0057` were narrowed again. `src/platform_web/web_platform_services.*`
   now owns the concrete Web `PlatformServices` implementation and
diff --git a/docs/modernization/roadmap.md b/docs/modernization/roadmap.md
index abf83857..4c1e816e 100644
--- a/docs/modernization/roadmap.md
+++ b/docs/modernization/roadmap.md
@@ -209,9 +209,10 @@ Latest slice:
   `windows_platform_services.cpp`.
 - `src/platform_windows/windows_platform_services.cpp` now keeps the VR/runtime
   adapter surface without also owning the async GL context pocket.
-- `src/platform_windows/windows_main_window_session.*` now owns the retained
-  Win32 main-window session fields (`HWND`, title buffer, sandbox flag)
-  instead of leaving them inside `windows_runtime_shell.cpp`.
+- `src/platform_windows/windows_main_window_session.*` now owns the explicit
+  Win32 `MainWindowSession` object (`HWND`, title buffer, sandbox flag) that
+  the runtime shell binds for the live session instead of leaving those fields
+  behind retained accessors inside `windows_runtime_shell.cpp`.
 - `src/platform_windows/windows_runtime_shell.cpp` now keeps Windows runtime
   ownership focused on `App` and tablet lifetime rather than also owning
   main-window session metadata.
diff --git a/docs/modernization/tasks.md b/docs/modernization/tasks.md
index b8030de2..dad4089e 100644
--- a/docs/modernization/tasks.md
+++ b/docs/modernization/tasks.md
@@ -211,8 +211,9 @@ Current slice:
   Win32 async GL context lock/swap state.
 - `src/platform_windows/windows_platform_services.cpp` no longer carries the
   retained async render-context pocket.
-- `src/platform_windows/windows_main_window_session.*` now owns the retained
-  Win32 main-window session fields.
+- `src/platform_windows/windows_main_window_session.*` now owns the explicit
+  Win32 `MainWindowSession` object that the runtime shell binds for the live
+  session.
 - `src/platform_windows/windows_runtime_shell.cpp` no longer carries the
   retained `HWND` / title-buffer / sandbox pocket.
 - `src/platform_legacy/legacy_platform_services.*` now takes Android storage
diff --git a/src/platform_windows/windows_bootstrap_helpers.cpp b/src/platform_windows/windows_bootstrap_helpers.cpp
index ca324a10..8fff1cc4 100644
--- a/src/platform_windows/windows_bootstrap_helpers.cpp
+++ b/src/platform_windows/windows_bootstrap_helpers.cpp
@@ -164,6 +164,7 @@ void setup_exception_handler(const App& app)
     BT_SetPreErrHandler([](INT_PTR nErrHandlerParam){
         const auto* app = reinterpret_cast<const App*>(nErrHandlerParam);
         auto* canvas_document = app && app->canvas ? app->canvas->m_canvas.get() : nullptr;
+        const auto* session = bound_main_window_session();
         if (canvas_document && canvas_document->m_unsaved)
         {
             auto t = std::time(nullptr);
@@ -177,7 +178,7 @@ void setup_exception_handler(const App& app)
             GetFullPathNameA(path.c_str(), MAX_PATH, abspath, NULL);
             static char message[4096];
             snprintf(message, sizeof(message), "File recovered in: %s", abspath);
-            MessageBoxA(retained_main_window_handle_ref(), message, "File Recovery", MB_OK | MB_ICONWARNING);
+            MessageBoxA(session ? session->handle : nullptr, message, "File Recovery", MB_OK | MB_ICONWARNING);
         }
         LogRemote::I.file_close();
     }, reinterpret_cast<INT_PTR>(&app));
@@ -339,7 +340,8 @@ int read_WMI_info()
             if (get_int(clsObj, L"CodeIntegrityPolicyEnforcementStatus") > 0)
             {
                 LOG("SANDBOX DETECTED");
-                set_retained_main_window_sandboxed(true);
+                if (auto* session = bound_main_window_session())
+                    session->sandboxed = true;
             }
 
             SAFEARRAY *psaNames = NULL;
@@ -427,11 +429,11 @@ MainWindowStartupState initialize_main_window_startup_state(App& app)
     return startup;
 }
 
-void create_main_window(const MainWindowStartupState& startup, HWND& hWnd, HINSTANCE hInst, const wchar_t* window_title)
+void create_main_window(const MainWindowStartupState& startup, MainWindowSession& session, HINSTANCE hInst, const wchar_t* window_title)
 {
     const int window_width = startup.client_rect.right - startup.client_rect.left;
     const int window_height = startup.client_rect.bottom - startup.client_rect.top;
-    hWnd = CreateWindow(
+    session.handle = CreateWindow(
         startup.window_class.lpszClassName,
         window_title,
         startup.window_style,
@@ -492,7 +494,11 @@ pp::renderer::gl::OpenGlRuntimeInfo log_runtime_info()
     return runtime_info;
 }
 
-bool upgrade_to_core_gl_context(const MainWindowStartupState& startup, HWND& hWnd, HINSTANCE hInst, const wchar_t* window_title, OpenGlWindowContext& context)
+bool upgrade_to_core_gl_context(
+    const MainWindowStartupState& startup,
+    MainWindowSession& session,
+    HINSTANCE hInst,
+    OpenGlWindowContext& context)
 {
     if (!GLAD_WGL_ARB_create_context)
     {
@@ -506,11 +512,10 @@ bool upgrade_to_core_gl_context(const MainWindowStartupState& startup, HWND& hWn
 
     wglMakeCurrent(NULL, NULL);
     wglDeleteContext(context.render_context);
-    DestroyWindow(hWnd);
+    DestroyWindow(session.handle);
+    create_main_window(startup, session, hInst, session.title);
 
-    create_main_window(startup, hWnd, hInst, window_title);
-
-    context.device_context = GetDC(hWnd);
+    context.device_context = GetDC(session.handle);
     int pixel_format = 0;
     wglChoosePixelFormatARB(
         context.device_context,
@@ -529,12 +534,12 @@ bool upgrade_to_core_gl_context(const MainWindowStartupState& startup, HWND& hWn
     return true;
 }
 
-MainStartupResult initialize_main_window_and_gl(MainWindowStartupState& startup, HWND& hWnd, HINSTANCE hInst, wchar_t* window_title, OpenGlWindowContext& context)
+MainStartupResult initialize_main_window_and_gl(MainWindowStartupState& startup, MainWindowSession& session, HINSTANCE hInst, OpenGlWindowContext& context)
 {
-    create_main_window(startup, hWnd, hInst, L"PanoPainter");
+    create_main_window(startup, session, hInst, L"PanoPainter");
     initialize_pixel_format_descriptor(startup.pixel_format);
 
-    context.device_context = GetDC(hWnd);
+    context.device_context = GetDC(session.handle);
     const int pixel_format = ChoosePixelFormat(context.device_context, &startup.pixel_format);
     SetPixelFormat(context.device_context, pixel_format, &startup.pixel_format);
     context.render_context = wglCreateContext(context.device_context);
@@ -553,13 +558,13 @@ MainStartupResult initialize_main_window_and_gl(MainWindowStartupState& startup,
 
     const auto renderer_name = std::string(context.runtime_info.renderer != nullptr ? context.runtime_info.renderer : "");
     swprintf_s(
-        window_title,
+        session.title,
         512,
         L"PanoPainter %s (%s)",
         g_version_number_w,
         str2wstr(renderer_name).c_str());
 
-    if (!upgrade_to_core_gl_context(startup, hWnd, hInst, window_title, context))
+    if (!upgrade_to_core_gl_context(startup, session, hInst, context))
         return MainStartupResult::MissingCoreContextSupport;
 
     return MainStartupResult::Ok;
diff --git a/src/platform_windows/windows_bootstrap_helpers.h b/src/platform_windows/windows_bootstrap_helpers.h
index cfddeda6..a1c5bda9 100644
--- a/src/platform_windows/windows_bootstrap_helpers.h
+++ b/src/platform_windows/windows_bootstrap_helpers.h
@@ -5,6 +5,7 @@
 
 #include <string>
 
+#include "platform_windows/windows_main_window_session.h"
 #include "renderer_gl/opengl_capabilities.h"
 
 class App;
@@ -52,11 +53,11 @@ void ensure_runtime_data_directory();
 void setup_exception_handler(const App& app);
 int run_winmain_entry(int (*entry_point)(int, char**));
 MainWindowStartupState initialize_main_window_startup_state(App& app);
-void create_main_window(const MainWindowStartupState& startup, HWND& hWnd, HINSTANCE hInst, const wchar_t* window_title);
+void create_main_window(const MainWindowStartupState& startup, MainWindowSession& session, HINSTANCE hInst, const wchar_t* window_title);
 void initialize_pixel_format_descriptor(PIXELFORMATDESCRIPTOR& pixel_format);
 bool load_glad_entry_points(HDC device_context);
 pp::renderer::gl::OpenGlRuntimeInfo log_runtime_info();
-bool upgrade_to_core_gl_context(const MainWindowStartupState& startup, HWND& hWnd, HINSTANCE hInst, const wchar_t* window_title, OpenGlWindowContext& context);
-MainStartupResult initialize_main_window_and_gl(MainWindowStartupState& startup, HWND& hWnd, HINSTANCE hInst, wchar_t* window_title, OpenGlWindowContext& context);
+bool upgrade_to_core_gl_context(const MainWindowStartupState& startup, MainWindowSession& session, HINSTANCE hInst, OpenGlWindowContext& context);
+MainStartupResult initialize_main_window_and_gl(MainWindowStartupState& startup, MainWindowSession& session, HINSTANCE hInst, OpenGlWindowContext& context);
 
 }
diff --git a/src/platform_windows/windows_lifecycle_shell.cpp b/src/platform_windows/windows_lifecycle_shell.cpp
index c88db603..a1771d04 100644
--- a/src/platform_windows/windows_lifecycle_shell.cpp
+++ b/src/platform_windows/windows_lifecycle_shell.cpp
@@ -4,7 +4,6 @@
 
 #include "legacy_preference_storage.h"
 #include "platform_windows/windows_lifecycle_state.h"
-#include "platform_windows/windows_main_window_session.h"
 #include "platform_windows/windows_stylus_input.h"
 
 namespace pp::platform::windows {
diff --git a/src/platform_windows/windows_main_window_session.cpp b/src/platform_windows/windows_main_window_session.cpp
index 89e5c436..3be28578 100644
--- a/src/platform_windows/windows_main_window_session.cpp
+++ b/src/platform_windows/windows_main_window_session.cpp
@@ -5,43 +5,18 @@
 namespace pp::platform::windows {
 namespace {
 
-struct RetainedMainWindowSessionState final {
-    HWND handle{};
-    wchar_t title[512]{};
-    bool sandboxed = false;
-};
+MainWindowSession* retained_main_window_session = nullptr;
 
-[[nodiscard]] RetainedMainWindowSessionState& retained_main_window_session_state() noexcept
+}
+
+void bind_main_window_session(MainWindowSession* session) noexcept
 {
-    static RetainedMainWindowSessionState state;
-    return state;
+    retained_main_window_session = session;
 }
 
-}
-
-HWND& retained_main_window_handle_ref() noexcept
+MainWindowSession* bound_main_window_session() noexcept
 {
-    return retained_main_window_session_state().handle;
-}
-
-wchar_t* retained_main_window_title_buffer() noexcept
-{
-    return retained_main_window_session_state().title;
-}
-
-const wchar_t* retained_main_window_title() noexcept
-{
-    return retained_main_window_session_state().title;
-}
-
-bool retained_main_window_sandboxed() noexcept
-{
-    return retained_main_window_session_state().sandboxed;
-}
-
-void set_retained_main_window_sandboxed(bool sandboxed) noexcept
-{
-    retained_main_window_session_state().sandboxed = sandboxed;
+    return retained_main_window_session;
 }
 
 }
diff --git a/src/platform_windows/windows_main_window_session.h b/src/platform_windows/windows_main_window_session.h
index 1fae17a5..30b7c27c 100644
--- a/src/platform_windows/windows_main_window_session.h
+++ b/src/platform_windows/windows_main_window_session.h
@@ -4,10 +4,13 @@
 
 namespace pp::platform::windows {
 
-[[nodiscard]] HWND& retained_main_window_handle_ref() noexcept;
-[[nodiscard]] wchar_t* retained_main_window_title_buffer() noexcept;
-[[nodiscard]] const wchar_t* retained_main_window_title() noexcept;
-[[nodiscard]] bool retained_main_window_sandboxed() noexcept;
-void set_retained_main_window_sandboxed(bool sandboxed) noexcept;
+struct MainWindowSession final {
+    HWND handle{};
+    wchar_t title[512]{};
+    bool sandboxed = false;
+};
+
+void bind_main_window_session(MainWindowSession* session) noexcept;
+[[nodiscard]] MainWindowSession* bound_main_window_session() noexcept;
 
 }
diff --git a/src/platform_windows/windows_platform_services.cpp b/src/platform_windows/windows_platform_services.cpp
index 00c5d6e5..4f44c08f 100644
--- a/src/platform_windows/windows_platform_services.cpp
+++ b/src/platform_windows/windows_platform_services.cpp
@@ -34,12 +34,18 @@ HWND pp_windows_main_window_handle();
 
 HWND pp_windows_main_window_handle()
 {
-    return pp::platform::windows::retained_main_window_handle_ref();
+    if (const auto* session = pp::platform::windows::bound_main_window_session())
+        return session->handle;
+    return nullptr;
 }
 
 void destroy_window()
 {
-    pp::platform::windows::enqueue_main_thread_task(std::packaged_task<void()>([hWnd = pp::platform::windows::retained_main_window_handle_ref()] {
+    const auto* session = pp::platform::windows::bound_main_window_session();
+    if (!session)
+        return;
+
+    pp::platform::windows::enqueue_main_thread_task(std::packaged_task<void()>([hWnd = session->handle] {
         pp::platform::windows::request_window_close(hWnd);
     }));
 }
@@ -71,9 +77,13 @@ void win32_update_stylus(float dt)
 
 void win32_update_fps(int frames)
 {
+    const auto* session = pp::platform::windows::bound_main_window_session();
+    if (!session)
+        return;
+
     pp::platform::windows::enqueue_main_thread_task(std::packaged_task<void()>([
-        hWnd = pp::platform::windows::retained_main_window_handle_ref(),
-        window_title = pp::platform::windows::retained_main_window_title(),
+        hWnd = session->handle,
+        window_title = session->title,
         vr = &pp::platform::windows::platform_vr_state(),
         frames] {
         pp::platform::windows::update_window_fps(hWnd, window_title, *vr, frames);
@@ -82,9 +92,13 @@ void win32_update_fps(int frames)
 
 bool win32_vr_start()
 {
+    const auto* session = pp::platform::windows::bound_main_window_session();
+    if (!session)
+        return false;
+
     return pp::platform::windows::start_window_vr(
         pp::platform::windows::platform_vr_state(),
-        pp::platform::windows::retained_main_window_sandboxed());
+        session->sandboxed);
 }
 
 void win32_vr_stop()
@@ -94,7 +108,8 @@ void win32_vr_stop()
 
 void win32_save_window_state()
 {
-    pp::platform::windows::save_window_preferences(pp::platform::windows::retained_main_window_handle_ref());
+    if (const auto* session = pp::platform::windows::bound_main_window_session())
+        pp::platform::windows::save_window_preferences(session->handle);
 }
 
 namespace pp::platform::windows {
diff --git a/src/platform_windows/windows_runtime_flow.cpp b/src/platform_windows/windows_runtime_flow.cpp
index 85fbdfe1..5502ecaf 100644
--- a/src/platform_windows/windows_runtime_flow.cpp
+++ b/src/platform_windows/windows_runtime_flow.cpp
@@ -8,11 +8,12 @@ namespace pp::platform::windows {
 
 void run_bound_main_window_runtime(
     const MainWindowStartupState& startup,
+    MainWindowSession& session,
     bool start_in_vr,
     HINSTANCE instance,
     SplashScreen& splash)
 {
-    run_main_window_runtime_session(startup, start_in_vr, instance, splash);
+    run_main_window_runtime_session(startup, session, start_in_vr, instance, splash);
 }
 
 }
diff --git a/src/platform_windows/windows_runtime_flow.h b/src/platform_windows/windows_runtime_flow.h
index e25fdc31..b84638ea 100644
--- a/src/platform_windows/windows_runtime_flow.h
+++ b/src/platform_windows/windows_runtime_flow.h
@@ -1,12 +1,14 @@
 #pragma once
 
 #include "platform_windows/windows_bootstrap_helpers.h"
+#include "platform_windows/windows_main_window_session.h"
 #include "platform_windows/windows_splash.h"
 
 namespace pp::platform::windows {
 
 void run_bound_main_window_runtime(
     const MainWindowStartupState& startup,
+    MainWindowSession& session,
     bool start_in_vr,
     HINSTANCE instance,
     SplashScreen& splash);
diff --git a/src/platform_windows/windows_runtime_session.cpp b/src/platform_windows/windows_runtime_session.cpp
index aa20c712..3fc8f295 100644
--- a/src/platform_windows/windows_runtime_session.cpp
+++ b/src/platform_windows/windows_runtime_session.cpp
@@ -108,26 +108,26 @@ void shutdown_main_window_runtime(const MainWindowStartupState& startup, HINSTAN
 
 }
 
-void run_main_window_runtime_session(const MainWindowStartupState& startup, bool start_in_vr, HINSTANCE instance, SplashScreen& splash)
+void run_main_window_runtime_session(const MainWindowStartupState& startup, MainWindowSession& session, bool start_in_vr, HINSTANCE instance, SplashScreen& splash)
 {
     auto* app = bound_app();
-    register_touch_window(retained_main_window_handle_ref());
+    register_touch_window(session.handle);
 
     wglMakeCurrent(NULL, NULL);
 
     initialize_runtime_threads();
     install_debug_gl_callbacks();
-    initialize_wintab(retained_main_window_handle_ref(), retained_main_window_sandboxed());
-    set_main_window_icon(retained_main_window_handle_ref());
+    initialize_wintab(session.handle, session.sandboxed);
+    set_main_window_icon(session.handle);
 
     app->ui_sync();
-    restore_window_placement(retained_main_window_handle_ref(), startup.show_command);
+    restore_window_placement(session.handle, startup.show_command);
 
     if (start_in_vr)
         app->vr_start();
 
     LOG("show main window");
-    SetForegroundWindow(retained_main_window_handle_ref());
+    SetForegroundWindow(session.handle);
 
     splash.dismiss();
 
diff --git a/src/platform_windows/windows_runtime_session.h b/src/platform_windows/windows_runtime_session.h
index bf6dce68..0a8b852e 100644
--- a/src/platform_windows/windows_runtime_session.h
+++ b/src/platform_windows/windows_runtime_session.h
@@ -1,12 +1,14 @@
 #pragma once
 
 #include "platform_windows/windows_bootstrap_helpers.h"
+#include "platform_windows/windows_main_window_session.h"
 #include "platform_windows/windows_splash.h"
 
 namespace pp::platform::windows {
 
 void run_main_window_runtime_session(
     const MainWindowStartupState& startup,
+    MainWindowSession& session,
     bool start_in_vr,
     HINSTANCE instance,
     SplashScreen& splash);
diff --git a/src/platform_windows/windows_runtime_shell.cpp b/src/platform_windows/windows_runtime_shell.cpp
index 00b379ab..022a3097 100644
--- a/src/platform_windows/windows_runtime_shell.cpp
+++ b/src/platform_windows/windows_runtime_shell.cpp
@@ -16,6 +16,8 @@ namespace pp::platform::windows {
 int run_main_application(int argc, char** argv)
 {
     const auto instance = GetModuleHandle(NULL);
+    auto session = MainWindowSession {};
+    bind_main_window_session(&session);
 
     auto& app = initialize_bound_app_runtime();
     app.set_platform_services(&pp::platform::windows::platform_services());
@@ -43,18 +45,19 @@ int run_main_application(int argc, char** argv)
     auto context = pp::platform::windows::OpenGlWindowContext {};
     switch (pp::platform::windows::initialize_main_window_and_gl(
         startup,
-        retained_main_window_handle_ref(),
+        session,
         instance,
-        retained_main_window_title_buffer(),
         context))
     {
     case pp::platform::windows::MainStartupResult::Ok:
         break;
     case pp::platform::windows::MainStartupResult::GladLoadFailure:
         release_bound_app();
+        bind_main_window_session(nullptr);
         return 0;
     case pp::platform::windows::MainStartupResult::MissingCoreContextSupport:
         release_bound_app();
+        bind_main_window_session(nullptr);
         return -1;
     }
 
@@ -69,6 +72,7 @@ int run_main_application(int argc, char** argv)
             app.initShaders();
             app.cmd_convert(argv[2], argv[3]);
             release_bound_app();
+            bind_main_window_session(nullptr);
             return 0;
         case const_hash("-vrmode"):
             start_in_vr = true;
@@ -78,8 +82,9 @@ int run_main_application(int argc, char** argv)
         }
     }
 
-    pp::platform::windows::run_bound_main_window_runtime(startup, start_in_vr, instance, splash);
+    pp::platform::windows::run_bound_main_window_runtime(startup, session, start_in_vr, instance, splash);
     release_bound_app();
+    bind_main_window_session(nullptr);
     return 0;
 }
 
diff --git a/src/platform_windows/windows_window_shell.cpp b/src/platform_windows/windows_window_shell.cpp
index 7378bc5f..3484f145 100644
--- a/src/platform_windows/windows_window_shell.cpp
+++ b/src/platform_windows/windows_window_shell.cpp
@@ -5,7 +5,6 @@
 #include "app.h"
 #include "platform_windows/windows_lifecycle_state.h"
 #include "platform_windows/windows_lifecycle_shell.h"
-#include "platform_windows/windows_main_window_session.h"
 #include "platform_windows/windows_platform_services.h"
 #include "platform_windows/windows_runtime_shell.h"
 #include "platform_windows/windows_stylus_input.h"