fix(windows): fix setting themes and maintain preferred theme when th…

…e system changes its theme (#844)

closes #840

.changes/windows-themes.md

@@ -0,0 +1,5 @@
+---
+"tao": "patch"
+---
+
+On Windows, fix `WindowBuilder::with_theme` has no effect when forcing light theme on a dark mode system.

src/platform_impl/windows/dark_mode.rs

@@ -7,11 +7,11 @@ use once_cell::sync::Lazy;
 /// This is a simple implementation of support for Windows Dark Mode,
 /// which is inspired by the solution in https://github.com/ysc3839/win32-darkmode
 use windows::{
-  core::{s, w, PCSTR, PSTR},
+  core::{s, w, PCSTR, PCWSTR, PSTR},
   Win32::{
     Foundation::{BOOL, HANDLE, HMODULE, HWND},
     System::{LibraryLoader::*, SystemInformation::OSVERSIONINFOW},
-    UI::{Accessibility::*, WindowsAndMessaging::*},
+    UI::{Accessibility::*, Controls::SetWindowTheme, WindowsAndMessaging::*},
   },
 };
 
@@ -59,7 +59,7 @@ static DARK_MODE_SUPPORTED: Lazy<bool> = Lazy::new(|| {
 });
 
 /// Attempts to set dark mode for the app
-pub fn try_app_theme(preferred_theme: Option<Theme>) -> Theme {
+pub fn try_app_theme(preferred_theme: Option<Theme>) {
   if *DARK_MODE_SUPPORTED {
     let is_dark_mode = match preferred_theme {
       Some(theme) => theme == Theme::Dark,
@@ -68,12 +68,6 @@ pub fn try_app_theme(preferred_theme: Option<Theme>) -> Theme {
 
     allow_dark_mode_for_app(is_dark_mode);
     refresh_immersive_color_policy_state();
-    match is_dark_mode {
-      true => Theme::Dark,
-      false => Theme::Light,
-    }
-  } else {
-    Theme::Light
   }
 }
 
@@ -160,14 +154,19 @@ pub fn try_window_theme(hwnd: HWND, preferred_theme: Option<Theme>) -> Theme {
       None => should_use_dark_mode(),
     };
 
-    let theme = if is_dark_mode {
-      Theme::Dark
-    } else {
-      Theme::Light
+    let theme = match is_dark_mode {
+      true => Theme::Dark,
+      false => Theme::Light,
+    };
+
+    let theme_name = match theme {
+      Theme::Dark => w!("DarkMode_Explorer"),
+      Theme::Light => w!(""),
     };
+    let _ = unsafe { SetWindowTheme(hwnd, theme_name, PCWSTR::null()) };
 
     allow_dark_mode_for_window(hwnd, is_dark_mode);
-    refresh_titlebar_theme_color(hwnd);
+    refresh_titlebar_theme_color(hwnd, is_dark_mode);
 
     theme
   } else {
@@ -197,29 +196,6 @@ fn allow_dark_mode_for_window(hwnd: HWND, is_dark_mode: bool) {
   }
 }
 
-fn is_dark_mode_allowed_for_window(hwnd: HWND) -> bool {
-  const UXTHEME_ISDARKMODEALLOWEDFORWINDOW_ORDINAL: u16 = 137;
-  type IsDarkModeAllowedForWindow = unsafe extern "system" fn(HWND) -> bool;
-  static IS_DARK_MODE_ALLOWED_FOR_WINDOW: Lazy<Option<IsDarkModeAllowedForWindow>> =
-    Lazy::new(|| unsafe {
-      if HUXTHEME.is_invalid() {
-        return None;
-      }
-
-      GetProcAddress(
-        *HUXTHEME,
-        PCSTR::from_raw(UXTHEME_ISDARKMODEALLOWEDFORWINDOW_ORDINAL as usize as *mut _),
-      )
-      .map(|handle| std::mem::transmute(handle))
-    });
-
-  if let Some(_is_dark_mode_allowed_for_window) = *IS_DARK_MODE_ALLOWED_FOR_WINDOW {
-    unsafe { _is_dark_mode_allowed_for_window(hwnd) }
-  } else {
-    false
-  }
-}
-
 type SetWindowCompositionAttribute =
   unsafe extern "system" fn(HWND, *mut WINDOWCOMPOSITIONATTRIBDATA) -> BOOL;
 static SET_WINDOW_COMPOSITION_ATTRIBUTE: Lazy<Option<SetWindowCompositionAttribute>> =
@@ -234,9 +210,9 @@ struct WINDOWCOMPOSITIONATTRIBDATA {
   cbData: usize,
 }
 
-fn refresh_titlebar_theme_color(hwnd: HWND) {
-  let dark = should_use_dark_mode() && is_dark_mode_allowed_for_window(hwnd);
-  let mut is_dark_mode_bigbool: BOOL = dark.into();
+fn refresh_titlebar_theme_color(hwnd: HWND, is_dark_mode: bool) {
+  // SetWindowCompositionAttribute needs a bigbool (i32), not bool.
+  let mut is_dark_mode_bigbool: i32 = is_dark_mode.into();
 
   if let Some(ver) = *WIN10_BUILD_VERSION {
     if ver < 18362 {

src/platform_impl/windows/event_loop.rs

@@ -107,6 +107,7 @@ pub(crate) struct SubclassInput<T: 'static> {
   pub _file_drop_handler: Option<IDropTarget>,
   pub subclass_removed: Cell<bool>,
   pub recurse_depth: Cell<u32>,
+  pub event_loop_preferred_theme: Option<Theme>,
 }
 
 impl<T> SubclassInput<T> {
@@ -161,7 +162,7 @@ impl Default for PlatformSpecificEventLoopAttributes {
 pub struct EventLoopWindowTarget<T: 'static> {
   thread_id: u32,
   thread_msg_target: HWND,
-  pub(crate) theme: Theme,
+  pub(crate) preferred_theme: Option<Theme>,
   pub(crate) runner_shared: EventLoopRunnerShared<T>,
 }
 
@@ -184,7 +185,7 @@ impl<T: 'static> EventLoop<T> {
 
     let thread_msg_target = create_event_target_window();
 
-    let theme = try_app_theme(attributes.preferred_theme);
+    try_app_theme(attributes.preferred_theme);
 
     let send_thread_msg_target = thread_msg_target;
     thread::spawn(move || wait_thread(thread_id, send_thread_msg_target));
@@ -202,7 +203,7 @@ impl<T: 'static> EventLoop<T> {
           thread_id,
           thread_msg_target,
           runner_shared,
-          theme,
+          preferred_theme: attributes.preferred_theme,
         },
         _marker: PhantomData,
       },
@@ -2056,7 +2057,10 @@ unsafe fn public_window_callback_inner<T: 'static>(
       let preferred_theme = subclass_input.window_state.lock().preferred_theme;
 
       if preferred_theme.is_none() {
-        let new_theme = try_window_theme(window, preferred_theme);
+        let new_theme = try_window_theme(
+          window,
+          preferred_theme.or(subclass_input.event_loop_preferred_theme),
+        );
         let mut window_state = subclass_input.window_state.lock();
 
         if window_state.current_theme != new_theme {

src/platform_impl/windows/mod.rs

@@ -21,7 +21,7 @@ pub(crate) use self::{
 
 pub use self::icon::WinIcon as PlatformIcon;
 
-use crate::{event::DeviceId as RootDeviceId, icon::Icon, keyboard::Key, window::Theme};
+use crate::{event::DeviceId as RootDeviceId, icon::Icon, keyboard::Key};
 mod keycode;
 
 #[non_exhaustive]
@@ -41,7 +41,6 @@ pub struct PlatformSpecificWindowBuilderAttributes {
   pub window_classname: String,
   pub no_redirection_bitmap: bool,
   pub drag_and_drop: bool,
-  pub preferred_theme: Option<Theme>,
   pub decoration_shadow: bool,
   pub rtl: bool,
 }
@@ -54,7 +53,6 @@ impl Default for PlatformSpecificWindowBuilderAttributes {
       taskbar_icon: None,
       no_redirection_bitmap: false,
       drag_and_drop: true,
-      preferred_theme: None,
       skip_taskbar: false,
       window_classname: "Window Class".to_string(),
       decoration_shadow: true,

src/platform_impl/windows/window.rs

@@ -122,6 +122,7 @@ impl Window {
           _file_drop_handler: file_drop_handler,
           subclass_removed: Cell::new(false),
           recurse_depth: Cell::new(0),
+          event_loop_preferred_theme: event_loop.preferred_theme,
         };
 
         event_loop::subclass_window(win.window.0, subclass_input);
@@ -1112,7 +1113,7 @@ unsafe fn init<T: 'static>(
   // window for the first time).
   let current_theme = try_window_theme(
     real_window.0,
-    attributes.preferred_theme.or(Some(event_loop.theme)),
+    attributes.preferred_theme.or(event_loop.preferred_theme),
   );
 
   let window_state = {
@@ -1121,7 +1122,7 @@ unsafe fn init<T: 'static>(
       None,
       scale_factor,
       current_theme,
-      pl_attribs.preferred_theme,
+      attributes.preferred_theme,
     );
     let window_state = Arc::new(Mutex::new(window_state));
     WindowState::set_window_flags(window_state.lock(), real_window.0, |f| *f = window_flags);