bug#51404: Support system dark mode on Windows 10

[Vince Salvino <salvino@coderedcorp.com>]

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Update: I managed to get my hands on an 1809 system and was able to get dark mode working there as well. As far as I can tell 1809 is the absolute minimum as that is when this setting and dark mode Explorer were introduced into Windows.

The advantage is that this will now work on Windows Server 2019 and Windows LTSC 2019, which some folks may be limited to as those are the latest Server and LTSC releases.

Attached patch includes the complete change, with relevant notes etc.

Vince Salvino


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From 856600cd86ce6305023c7dbb8a41649c80d305e0 Mon Sep 17 00:00:00 2001
From: Vince Salvino <salvino@coderedcorp.com>
Date: Wed, 27 Oct 2021 17:32:09 -0400
Subject: [PATCH] Support system dark mode on Windows 10 version 1809 and
 higher.

---
 doc/emacs/msdos.texi |  7 ++++
 etc/NEWS             |  8 ++++
 src/w32.c            | 23 +++++++----
 src/w32.h            |  5 ++-
 src/w32fns.c         | 91 +++++++++++++++++++++++++++++++++++++++++++-
 5 files changed, 122 insertions(+), 12 deletions(-)

diff --git a/doc/emacs/msdos.texi b/doc/emacs/msdos.texi
index 0f8f429b3f..3c6c61613e 100644
--- a/doc/emacs/msdos.texi
+++ b/doc/emacs/msdos.texi
@@ -1181,6 +1181,13 @@ Windows Misc
 click-to-focus policy.
 @end ifnottex
 
+  On Windows 10 (version 1809 and higher) and Windows 11, Emacs title
+bars and scroll bars will follow the system Light or Dark mode,
+similar to other programs such as Explorer and Command Prompt. To
+change the color mode: Windows Settings > Personalization > Colors >
+Choose your color (or Choose your default app mode); then restart
+Emacs.
+
 @ifnottex
 @include msdos-xtra.texi
 @end ifnottex
diff --git a/etc/NEWS b/etc/NEWS
index e475a49b98..82d8048c23 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -73,6 +73,14 @@ Image specifiers can now use ':type webp'.
 *** 'display-buffer' now can set up the body size of the chosen window.
 For example, an alist entry as '(window-width . (body-columns . 40))'
 will make the body of the chosen window 40 columns wide.
+
+** MS-Windows
+
++++
+*** Supports dark mode on Windows 10 (version 1809 and higher) and Windows 11.
+Graphical frames now use the appropriate light or dark title bar and
+scroll bars, based on the user's Windows color settings.
+
 \f
 * Editing Changes in Emacs 29.1
 
diff --git a/src/w32.c b/src/w32.c
index 9fe698d28d..369e7ee4e1 100644
--- a/src/w32.c
+++ b/src/w32.c
@@ -2820,8 +2820,15 @@ sys_putenv (char *str)
 
 #define REG_ROOT "SOFTWARE\\GNU\\Emacs"
 
+/* Query a value from the Windows Registry (under HKCU and HKLM),
+   where `key` is the registry key, `name` is the name, and `lpdwtype`
+   is a pointer to the return value's type. `lpwdtype` can be NULL if
+   you do not care about the type.
+
+   Returns: pointer to the value, or null pointer if the key/name does
+   not exist. */
 LPBYTE
-w32_get_resource (const char *key, LPDWORD lpdwtype)
+w32_get_resource (const char *key, const char *name, LPDWORD lpdwtype)
 {
   LPBYTE lpvalue;
   HKEY hrootkey = NULL;
@@ -2830,13 +2837,13 @@ w32_get_resource (const char *key, LPDWORD lpdwtype)
   /* Check both the current user and the local machine to see if
      we have any resources.  */
 
-  if (RegOpenKeyEx (HKEY_CURRENT_USER, REG_ROOT, 0, KEY_READ, &hrootkey) == ERROR_SUCCESS)
+  if (RegOpenKeyEx (HKEY_CURRENT_USER, key, 0, KEY_READ, &hrootkey) == ERROR_SUCCESS)
     {
       lpvalue = NULL;
 
-      if (RegQueryValueEx (hrootkey, key, NULL, NULL, NULL, &cbData) == ERROR_SUCCESS
+      if (RegQueryValueEx (hrootkey, name, NULL, NULL, NULL, &cbData) == ERROR_SUCCESS
 	  && (lpvalue = xmalloc (cbData)) != NULL
-	  && RegQueryValueEx (hrootkey, key, NULL, lpdwtype, lpvalue, &cbData) == ERROR_SUCCESS)
+	  && RegQueryValueEx (hrootkey, name, NULL, lpdwtype, lpvalue, &cbData) == ERROR_SUCCESS)
 	{
           RegCloseKey (hrootkey);
 	  return (lpvalue);
@@ -2847,13 +2854,13 @@ w32_get_resource (const char *key, LPDWORD lpdwtype)
       RegCloseKey (hrootkey);
     }
 
-  if (RegOpenKeyEx (HKEY_LOCAL_MACHINE, REG_ROOT, 0, KEY_READ, &hrootkey) == ERROR_SUCCESS)
+  if (RegOpenKeyEx (HKEY_LOCAL_MACHINE, key, 0, KEY_READ, &hrootkey) == ERROR_SUCCESS)
     {
       lpvalue = NULL;
 
-      if (RegQueryValueEx (hrootkey, key, NULL, NULL, NULL, &cbData) == ERROR_SUCCESS
+      if (RegQueryValueEx (hrootkey, name, NULL, NULL, NULL, &cbData) == ERROR_SUCCESS
 	  && (lpvalue = xmalloc (cbData)) != NULL
-	  && RegQueryValueEx (hrootkey, key, NULL, lpdwtype, lpvalue, &cbData) == ERROR_SUCCESS)
+	  && RegQueryValueEx (hrootkey, name, NULL, lpdwtype, lpvalue, &cbData) == ERROR_SUCCESS)
 	{
           RegCloseKey (hrootkey);
 	  return (lpvalue);
@@ -3077,7 +3084,7 @@ #define SET_ENV_BUF_SIZE (4 * MAX_PATH)	/* to cover EMACSLOADPATH */
 	    int dont_free = 0;
 	    char bufc[SET_ENV_BUF_SIZE];
 
-	    if ((lpval = w32_get_resource (env_vars[i].name, &dwType)) == NULL
+	    if ((lpval = w32_get_resource (REG_ROOT, env_vars[i].name, &dwType)) == NULL
 		/* Also ignore empty environment variables.  */
 		|| *lpval == 0)
 	      {
diff --git a/src/w32.h b/src/w32.h
index ffa145b148..ec0f37123e 100644
--- a/src/w32.h
+++ b/src/w32.h
@@ -161,8 +161,9 @@ #define FILE_SERIAL             0x0800
 extern void reset_standard_handles (int in, int out,
 				    int err, HANDLE handles[4]);
 
-/* Return the string resource associated with KEY of type TYPE.  */
-extern LPBYTE w32_get_resource (const char * key, LPDWORD type);
+/* Query Windows Registry and return the resource associated
+   associated with KEY and NAME of type TYPE.  */
+extern LPBYTE w32_get_resource (const char * key, const char * name, LPDWORD type);
 
 extern void release_listen_threads (void);
 extern void init_ntproc (int);
diff --git a/src/w32fns.c b/src/w32fns.c
index 14d1154a2b..bcf0f50c6a 100644
--- a/src/w32fns.c
+++ b/src/w32fns.c
@@ -73,6 +73,18 @@ #define _WIN32_WINNT 0x0600
 #include <imm.h>
 #include <windowsx.h>
 
+/*
+  Internal/undocumented constants for Windows Dark mode.
+  See: https://github.com/microsoft/WindowsAppSDK/issues/41
+*/
+#define DARK_MODE_APP_NAME L"DarkMode_Explorer"
+/* For Windows 10 version 1809, 1903, 1909. */
+#define DWMWA_USE_IMMERSIVE_DARK_MODE_OLD 19
+/* For Windows 10 version 2004 and higher, and Windows 11. */
+#ifndef DWMWA_USE_IMMERSIVE_DARK_MODE
+#define DWMWA_USE_IMMERSIVE_DARK_MODE 20
+#endif
+
 #ifndef FOF_NO_CONNECTED_ELEMENTS
 #define FOF_NO_CONNECTED_ELEMENTS 0x2000
 #endif
@@ -185,6 +197,11 @@ DECLARE_HANDLE(HMONITOR);
 typedef HRESULT (WINAPI *SetThreadDescription_Proc)
   (HANDLE hThread, PCWSTR lpThreadDescription);
 
+typedef HRESULT (WINAPI * SetWindowTheme_Proc)
+  (IN HWND hwnd, IN LPCWSTR pszSubAppName, IN LPCWSTR pszSubIdList);
+typedef HRESULT (WINAPI * DwmSetWindowAttribute_Proc)
+  (HWND hwnd, DWORD dwAttribute, IN LPCVOID pvAttribute, DWORD cbAttribute);
+
 TrackMouseEvent_Proc track_mouse_event_fn = NULL;
 ImmGetCompositionString_Proc get_composition_string_fn = NULL;
 ImmGetContext_Proc get_ime_context_fn = NULL;
@@ -199,6 +216,8 @@ DECLARE_HANDLE(HMONITOR);
 GetTitleBarInfo_Proc get_title_bar_info_fn = NULL;
 IsDebuggerPresent_Proc is_debugger_present = NULL;
 SetThreadDescription_Proc set_thread_description = NULL;
+SetWindowTheme_Proc SetWindowTheme_fn = NULL;
+DwmSetWindowAttribute_Proc DwmSetWindowAttribute_fn = NULL;
 
 extern AppendMenuW_Proc unicode_append_menu;
 
@@ -252,6 +271,9 @@ #define MENU_FREE_DELAY 1000
 int w32_minor_version;
 int w32_build_number;
 
+/* If the OS is set to use dark mode. */
+BOOL w32_darkmode = FALSE;
+
 /* Distinguish between Windows NT and Windows 95.  */
 int os_subtype;
 
@@ -2279,10 +2301,36 @@ w32_init_class (HINSTANCE hinst)
     }
 }
 
+/* Applies the Windows system theme (light or dark) to a window handle. */
+static void
+w32_applytheme (HWND hwnd)
+{
+  if (w32_darkmode)
+    {
+      /* Set window theme to that of a built-in Windows app (Explorer)
+	 because it has dark scroll bars and other UI elements. */
+      if (SetWindowTheme_fn)
+	{
+	  SetWindowTheme_fn (hwnd, DARK_MODE_APP_NAME, NULL);
+	}
+      /* Set the titlebar to system dark mode. */
+      if (DwmSetWindowAttribute_fn)
+	{
+	  /* Windows 10 version 2004 and up, Windows 11. */
+	  DWORD attr = DWMWA_USE_IMMERSIVE_DARK_MODE;
+	  /* Windows 10 older than 2004. */
+	  if (w32_build_number < 19041)
+	    attr = DWMWA_USE_IMMERSIVE_DARK_MODE_OLD;
+	  DwmSetWindowAttribute_fn
+	    (hwnd, attr, &w32_darkmode, sizeof(w32_darkmode));
+	}
+    }
+}
+
 static HWND
 w32_createvscrollbar (struct frame *f, struct scroll_bar * bar)
 {
-  return CreateWindow ("SCROLLBAR", "",
+  HWND hwnd = CreateWindow ("SCROLLBAR", "",
 		       /* Clip siblings so we don't draw over child
 			  frames.  Apparently this is not always
 			  sufficient so we also try to make bar windows
@@ -2291,12 +2339,15 @@ w32_createvscrollbar (struct frame *f, struct scroll_bar * bar)
 		       /* Position and size of scroll bar.  */
 		       bar->left, bar->top, bar->width, bar->height,
 		       FRAME_W32_WINDOW (f), NULL, hinst, NULL);
+  if (hwnd)
+    w32_applytheme (hwnd);
+  return hwnd;
 }
 
 static HWND
 w32_createhscrollbar (struct frame *f, struct scroll_bar * bar)
 {
-  return CreateWindow ("SCROLLBAR", "",
+  HWND hwnd = CreateWindow ("SCROLLBAR", "",
 		       /* Clip siblings so we don't draw over child
 			  frames.  Apparently this is not always
 			  sufficient so we also try to make bar windows
@@ -2305,6 +2356,9 @@ w32_createhscrollbar (struct frame *f, struct scroll_bar * bar)
 		       /* Position and size of scroll bar.  */
 		       bar->left, bar->top, bar->width, bar->height,
 		       FRAME_W32_WINDOW (f), NULL, hinst, NULL);
+  if (hwnd)
+    w32_applytheme (hwnd);
+  return hwnd;
 }
 
 static void
@@ -2390,6 +2444,9 @@ w32_createwindow (struct frame *f, int *coords)
       /* Enable drag-n-drop.  */
       DragAcceptFiles (hwnd, TRUE);
 
+      /* Enable system light/dark theme. */
+      w32_applytheme (hwnd);
+
       /* Do this to discard the default setting specified by our parent. */
       ShowWindow (hwnd, SW_HIDE);
 
@@ -11028,6 +11085,36 @@ globals_of_w32fns (void)
   set_thread_description = (SetThreadDescription_Proc)
     get_proc_addr (hm_kernel32, "SetThreadDescription");
 
+  /* Support OS dark mode on Windows 10 version 1809 and higher.
+     See `w32_applytheme` which uses appropriate APIs per version of Windows.
+     For future wretches who may need to understand Windows build numbers:
+     https://docs.microsoft.com/en-us/windows/release-health/release-information
+  */
+  if (w32_major_version >= 10 && w32_build_number >= 17763
+      && os_subtype == OS_SUBTYPE_NT)
+    {
+      /* Load dwmapi and uxtheme, which will be needed to set window themes. */
+      HMODULE dwmapi_lib = LoadLibrary("dwmapi.dll");
+      DwmSetWindowAttribute_fn = (DwmSetWindowAttribute_Proc)
+	get_proc_addr (dwmapi_lib, "DwmSetWindowAttribute");
+      HMODULE uxtheme_lib = LoadLibrary("uxtheme.dll");
+      SetWindowTheme_fn = (SetWindowTheme_Proc)
+	get_proc_addr (uxtheme_lib, "SetWindowTheme");
+
+      /* Check Windows Registry for system theme. DWORD set to 0 or 1.
+	 TODO: "Nice to have" would be to create a lisp setting (which
+	 defaults to this Windows Registry value), then read that lisp
+	 value here instead. This would allow the user to forcibly
+	 override the system theme (which is also user-configurable in
+	 Windows settings; see MS-Windows section in Emacs manual). */
+      LPBYTE val = w32_get_resource
+	("Software\\Microsoft\\Windows\\CurrentVersion\\Themes\\Personalize",
+	 "AppsUseLightTheme",
+	 NULL);
+      if (val && (DWORD)*val == 0)
+	w32_darkmode = TRUE;
+    }
+
   except_code = 0;
   except_addr = 0;
 #ifndef CYGWIN
-- 
2.33.0.windows.2