fix(windows): use registry value to detect dark mode (#1165)

* fix(windows): respect default app mode via registry in dark mode detection

* chore(windows): document Win11 issue with ShouldAppsUseDarkMode

.changes/fix-windows-dark-mode.md

@@ -0,0 +1,5 @@
+---
+"tao": patch
+---
+
+fix(windows): respect default app mode via registry in dark mode detection

Cargo.toml

@@ -79,6 +79,7 @@ windows-core = "0.61"
   "Win32_System_LibraryLoader",
   "Win32_System_Memory",
   "Win32_System_Ole",
+  "Win32_System_Registry",
   "Win32_System_SystemServices",
   "Win32_System_Threading",
   "Win32_System_WindowsProgramming",

src/platform_impl/windows/dark_mode.rs

@@ -9,9 +9,12 @@ use once_cell::sync::Lazy;
 use windows::{
   core::{s, w, BOOL, PCSTR, PSTR},
   Win32::{
-    Foundation::{HANDLE, HMODULE, HWND, LPARAM, WPARAM},
+    Foundation::{ERROR_SUCCESS, HANDLE, HMODULE, HWND, LPARAM, WPARAM},
     Graphics::Dwm::{DwmSetWindowAttribute, DWMWINDOWATTRIBUTE},
-    System::LibraryLoader::*,
+    System::{
+      LibraryLoader::*,
+      Registry::{RegGetValueW, HKEY_CURRENT_USER, RRF_RT_REG_DWORD},
+    },
     UI::{Accessibility::*, Input::KeyboardAndMouse::GetActiveWindow, WindowsAndMessaging::*},
   },
 };
@@ -199,6 +202,12 @@ fn should_use_dark_mode() -> bool {
 }
 
 fn should_apps_use_dark_mode() -> bool {
+  if let Some(apps_use_light_theme) = read_apps_use_light_theme() {
+    return !apps_use_light_theme;
+  }
+  // This undocumented method `ShouldAppsUseDarkMode` may return
+  // incorrect values on Windows 11.
+  // See https://github.com/tauri-apps/tao/pull/1165
   const UXTHEME_SHOULDAPPSUSEDARKMODE_ORDINAL: u16 = 132;
   type ShouldAppsUseDarkMode = unsafe extern "system" fn() -> bool;
   static SHOULD_APPS_USE_DARK_MODE: Lazy<Option<ShouldAppsUseDarkMode>> = Lazy::new(|| unsafe {
@@ -218,6 +227,27 @@ fn should_apps_use_dark_mode() -> bool {
     .unwrap_or(false)
 }
 
+fn read_apps_use_light_theme() -> Option<bool> {
+  let mut data: u32 = 0;
+  let mut data_size = std::mem::size_of::<u32>() as u32;
+  let status = unsafe {
+    RegGetValueW(
+      HKEY_CURRENT_USER,
+      w!(r"Software\Microsoft\Windows\CurrentVersion\Themes\Personalize"),
+      w!("AppsUseLightTheme"),
+      RRF_RT_REG_DWORD,
+      None,
+      Some(&mut data as *mut _ as _),
+      Some(&mut data_size),
+    )
+  };
+  if status == ERROR_SUCCESS {
+    Some(data != 0)
+  } else {
+    None
+  }
+}
+
 fn is_high_contrast() -> bool {
   const HCF_HIGHCONTRASTON: u32 = 1;