Support explicit switching both to and from dark mode in wxMSW

With these changes, wxApp::SetAppearance() and
wxApp::MSWEnableDarkMode() can change the dark/light mode at any time.
They can be called before or after windows are created, and can be
called more than once.

The widgets sample has new menu commands to demonstrate this.

Closes #26734.

include/wx/msw/app.h

@@ -46,7 +46,8 @@ public:
     enum
     {
         DarkMode_Auto   = 0,  // Use dark mode if the system is using it.
-        DarkMode_Always = 1   // Force using dark mode.
+        DarkMode_Always,      // Force using dark mode.
+        DarkMode_Never        // Force light mode.
     };
     bool
     MSWEnableDarkMode(int flags = 0, wxDarkModeSettings* settings = nullptr);

interface/wx/app.h

@@ -1439,10 +1439,11 @@ public:
         - wxTimePickerCtrl, wxDatePickerCtrl and wxCalendarCtrl don't support dark mode
           and use the same (light) background as by default in it.
 
-        @param flags Can include @c wxApp::DarkMode_Always to force enabling
-            dark mode for the application, even if the system doesn't use the
-            dark mode by default. Otherwise dark mode is only used if it is the
-            default mode for the applications on the current system.
+        @param flags Can be @c wxApp::DarkMode_Always to force dark mode
+            regardless of the system mode, @c wxApp::DarkMode_Never to likewise
+            force light mode, or @c wxApp::DarkMode_Auto to follow the system
+            mode. The constant @c wxApp::DarkMode_Never is available since
+            wxWidgets 3.3.4.
         @param settings If specified, allows to customize dark mode appearance.
             Please see wxDarkModeSettings documentation for more information.
 

samples/widgets/widgets.cpp

@@ -113,7 +113,11 @@ enum
     TextEntry_AutoCompleteKeyLength,
 
     TextEntry_SetHint,
-    TextEntry_End
+    TextEntry_End,
+
+    Theme_Light,
+    Theme_Dark,
+    Theme_System,
 };
 
 const wxChar *WidgetsCategories[MAX_PAGES] = {
@@ -248,6 +252,8 @@ protected:
     {
         event.Enable( CurrentPage()->GetTextEntry() != nullptr );
     }
+
+    void OnTheme(wxCommandEvent& event);
 #endif // wxUSE_MENUS
 
     // initialize the book: add all pages to it
@@ -375,6 +381,10 @@ wxBEGIN_EVENT_TABLE(WidgetsFrame, wxFrame)
                         WidgetsFrame::OnUpdateTextUI)
 
     EVT_MENU(wxID_EXIT, WidgetsFrame::OnExit)
+
+    EVT_MENU(Theme_Light, WidgetsFrame::OnTheme)
+    EVT_MENU(Theme_Dark, WidgetsFrame::OnTheme)
+    EVT_MENU(Theme_System, WidgetsFrame::OnTheme)
 #endif // wxUSE_MENUS
 wxEND_EVENT_TABLE()
 
@@ -584,6 +594,12 @@ WidgetsFrame::WidgetsFrame(const wxString& title)
 
     InitBook();
 
+    wxMenu* menuTheme = new wxMenu;
+    menuTheme->Append(Theme_System, "&System");
+    menuTheme->Append(Theme_Light, "&Light");
+    menuTheme->Append(Theme_Dark, "&Dark");
+    mbar->Append(menuTheme, "T&heme");
+
     // the lower one only has the log listbox and a button to clear it
 #if USE_LOG
     wxStaticBoxSizer *sizerDown = new wxStaticBoxSizer(wxVERTICAL, m_panel, "&Log window");
@@ -1299,6 +1315,25 @@ void WidgetsFrame::OnSetHint(wxCommandEvent& WXUNUSED(event))
     }
 }
 
+void WidgetsFrame::OnTheme(wxCommandEvent& event)
+{
+    wxApp::Appearance appearance;
+    switch ( event.GetId() )
+    {
+        case Theme_Dark:
+            appearance = wxApp::Appearance::Dark;
+            break;
+        case Theme_Light:
+            appearance = wxApp::Appearance::Light;
+            break;
+        default:
+            appearance = wxApp::Appearance::System;
+            break;
+    }
+    if ( wxTheApp->SetAppearance(appearance) == wxApp::AppearanceResult::Failure)
+        wxLogMessage("SetAppearance: failure");
+}
+
 #endif // wxUSE_MENUS
 
 // ----------------------------------------------------------------------------

src/msw/darkmode.cpp

@@ -151,6 +151,7 @@ PreferredAppMode gs_appMode = AppMode_Default;
 
 // The initial dark mode state.
 bool gs_wasActiveOnStartup  = false;
+bool gs_wasActiveOnStartupIsSet = false;
 
 // The return value for HasChanged().
 bool gs_hasChanged = false;
@@ -186,6 +187,7 @@ void (WINAPI *gs_RefreshImmersiveColorPolicyState)() = nullptr;
 bool (WINAPI *gs_ShouldAppsUseDarkMode)() = nullptr;
 bool (WINAPI *gs_AllowDarkModeForWindow)(HWND hwnd, bool allow) = nullptr;
 PreferredAppMode (WINAPI *gs_SetPreferredAppMode)(PreferredAppMode appMode) = nullptr;
+void (WINAPI *gs_FlushMenuThemes)() = nullptr;
 
 // Wrappers for the undocumented functions, to make sure we never dereference
 // a null function pointer.
@@ -218,6 +220,13 @@ PreferredAppMode SetPreferredAppMode(PreferredAppMode appMode)
     return gs_SetPreferredAppMode(appMode);
 }
 
+void FlushMenuThemes()
+{
+    if ( gs_FlushMenuThemes == nullptr )
+        return;
+    gs_FlushMenuThemes();
+}
+
 bool InitDarkMode()
 {
     // Enable dark mode for Windows 10 1903 (build 18362) and later.
@@ -238,7 +247,8 @@ bool InitDarkMode()
     return TryLoadByOrd(gs_RefreshImmersiveColorPolicyState, dllUxTheme, 104) &&
            TryLoadByOrd(gs_ShouldAppsUseDarkMode, dllUxTheme, 132) &&
            TryLoadByOrd(gs_AllowDarkModeForWindow, dllUxTheme, 133) &&
-           TryLoadByOrd(gs_SetPreferredAppMode, dllUxTheme, 135);
+           TryLoadByOrd(gs_SetPreferredAppMode, dllUxTheme, 135) &&
+           TryLoadByOrd(gs_FlushMenuThemes, dllUxTheme, 136);
 }
 
 // This function is only used in this file as it's more clear than using
@@ -279,8 +289,16 @@ bool wxApp::MSWEnableDarkMode(int flags, wxDarkModeSettings* settings)
     if ( !wxMSWImpl::InitDarkMode() )
         return false;
 
-    const PreferredAppMode mode = flags & DarkMode_Always ? AppMode_ForceDark
-                                                          : AppMode_AllowDark;
+    PreferredAppMode mode = PreferredAppMode::AppMode_AllowDark;
+    switch ( flags )
+    {
+        case DarkMode_Always:
+            mode = PreferredAppMode::AppMode_ForceDark;
+            break;
+        case DarkMode_Never:
+            mode = PreferredAppMode::AppMode_ForceLight;
+            break;
+    }
     const DWORD rc = wxMSWImpl::SetPreferredAppMode(mode);
 
     // It's supposed to return the old mode normally.
@@ -297,7 +315,15 @@ bool wxApp::MSWEnableDarkMode(int flags, wxDarkModeSettings* settings)
     wxMSWImpl::RefreshImmersiveColorPolicyState();
 
     gs_appMode = mode;
-    gs_wasActiveOnStartup = wxMSWDarkMode::IsActive();
+
+    // Initialize gs_wasActiveOnStartup only one time and only before a window
+    // is created. If we first reach here after a window was created, then
+    // dark mode was not active upon startup.
+    if ( !gs_wasActiveOnStartupIsSet && wxTopLevelWindows.IsEmpty() )
+    {
+        gs_wasActiveOnStartup = wxMSWDarkMode::IsActive();
+        gs_wasActiveOnStartupIsSet = true;
+    }
 
     // Set up the settings to use, allocating a default one if none specified.
     if ( !settings )
@@ -305,18 +331,17 @@ bool wxApp::MSWEnableDarkMode(int flags, wxDarkModeSettings* settings)
 
     wxDarkModeModule::SetSettings(settings);
 
+    // If we were called after the main window was created, propagate the
+    // change.
+    wxWindow* topWindow = GetTopWindow();
+    if ( topWindow != nullptr )
+        ::SendMessage(topWindow->GetHWND(), WM_SYSCOLORCHANGE, 0, 0);
+
     return true;
 }
 
 wxApp::AppearanceResult wxApp::SetAppearance(Appearance appearance)
 {
-    // We currently can't change the appearance of the existing windows because
-    // we initialize/create them differently depending on the mode value in a
-    // lot of places, so don't even try as we risk finishing with a horrible
-    // mix of light and dark mode elements.
-    if ( !wxTopLevelWindows.empty() || gs_appMode != AppMode_Default )
-        return AppearanceResult::CannotChange;
-
     int flags = 0;
     switch ( appearance )
     {
@@ -325,8 +350,8 @@ wxApp::AppearanceResult wxApp::SetAppearance(Appearance appearance)
             break;
 
         case Appearance::Light:
-            // Nothing to do, this is the default.
-            return AppearanceResult::Ok;
+            flags = DarkMode_Never;
+            break;
 
         case Appearance::Dark:
             flags = DarkMode_Always;
@@ -492,6 +517,19 @@ void ConfigureTLW(HWND hwnd)
         wxLogApiError("DwmSetWindowAttribute(USE_IMMERSIVE_DARK_MODE)", hr);
 
     wxMSWImpl::AllowDarkModeForWindow(hwnd, true);
+
+    // Purge cached theme data for existing menus so that menu colours are
+    // updated. This is needed when explicitly switching from dark to light.
+    wxMSWImpl::FlushMenuThemes();
+
+    // When explicitly switching modes, Windows 10 fails to repaint the title
+    // bar. Force it to repaint.
+    if ( wxGetWinVersion() < wxWinVersion_11 )
+    {
+        bool isActive = ::GetActiveWindow() == hwnd;
+        ::SendMessage(hwnd, WM_NCACTIVATE, !isActive, 0);
+        ::SendMessage(hwnd, WM_NCACTIVATE, isActive, 0);
+    }
 }
 
 void SetTheme(HWND hwnd, const wchar_t* themeName, const wchar_t* themeId)