Apply selected style to NSWindow elements (e.g. title bar) on macOS (#3986)

launcher/CMakeLists.txt

@@ -1178,6 +1178,13 @@ SET(LAUNCHER_SOURCES
     ui/instanceview/VisualGroup.h
 )
 
+if (APPLE)
+    set(LAUNCHER_SOURCES
+        ${LAUNCHER_SOURCES}
+        ui/themes/ThemeManager.mm
+    )
+endif()
+
 if (NOT Apple)
     set(LAUNCHER_SOURCES
     ${LAUNCHER_SOURCES}

launcher/ui/themes/ThemeManager.cpp

@@ -50,6 +50,11 @@ ThemeManager::ThemeManager()
     initializeCatPacks();
 }
 
+ThemeManager::~ThemeManager()
+{
+    stopSettingNewWindowColorsOnMac();
+}
+
 /// @brief Adds the Theme to the list of themes
 /// @param theme The Theme to add
 /// @return Theme ID
@@ -174,6 +179,15 @@ void ThemeManager::initializeWidgets()
     themeDebugLog() << "<> Widget themes initialized.";
 }
 
+#ifndef Q_OS_MACOS
+void ThemeManager::setTitlebarColorOnMac(WId windowId, QColor color)
+{}
+void ThemeManager::setTitlebarColorOfAllWindowsOnMac(QColor color)
+{}
+void ThemeManager::stopSettingNewWindowColorsOnMac()
+{}
+#endif
+
 QList<IconTheme*> ThemeManager::getValidIconThemes()
 {
     QList<IconTheme*> ret;
@@ -247,6 +261,7 @@ void ThemeManager::setApplicationTheme(const QString& name, bool initial)
         auto& theme = themeIter->second;
         themeDebugLog() << "applying theme" << theme->name();
         theme->apply(initial);
+        setTitlebarColorOfAllWindowsOnMac(qApp->palette().window().color());
 
         m_logColors = theme->logColorScheme();
     } else {

launcher/ui/themes/ThemeManager.h

@@ -39,6 +39,7 @@ inline auto themeWarningLog()
 class ThemeManager {
    public:
     ThemeManager();
+    ~ThemeManager();
 
     QList<IconTheme*> getValidIconThemes();
     QList<ITheme*> getValidApplicationThemes();
@@ -81,6 +82,17 @@ class ThemeManager {
     void initializeIcons();
     void initializeWidgets();
 
+    // On non-Mac systems, this is a no-op.
+    void setTitlebarColorOnMac(WId windowId, QColor color);
+    // This also will set the titlebar color of newly opened windows after this method is called.
+    // On non-Mac systems, this is a no-op.
+    void setTitlebarColorOfAllWindowsOnMac(QColor color);
+    // On non-Mac systems, this is a no-op.
+    void stopSettingNewWindowColorsOnMac();
+#ifdef Q_OS_MACOS
+    NSObject* m_windowTitlebarObserver = nullptr;
+#endif
+
     const QStringList builtinIcons{ "pe_colored", "pe_light", "pe_dark", "pe_blue",    "breeze_light", "breeze_dark",
                                     "OSX",        "iOS",      "flat",    "flat_white", "multimc" };
 };

launcher/ui/themes/ThemeManager.mm

@@ -0,0 +1,73 @@
+// SPDX-License-Identifier: GPL-3.0-only
+/*
+ *  Prism Launcher - Minecraft Launcher
+ *  Copyright (C) 2025 Kenneth Chew <79120643+kthchew@users.noreply.github.com>
+ *
+ *  This program is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, version 3.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+#include "ThemeManager.h"
+
+#include <AppKit/AppKit.h>
+
+void ThemeManager::setTitlebarColorOnMac(WId windowId, QColor color)
+{
+    if (windowId == 0) {
+        return;
+    }
+
+    NSView* view = (NSView*)windowId;
+    NSWindow* window = [view window];
+    window.titlebarAppearsTransparent = YES;
+    window.backgroundColor = [NSColor colorWithRed:color.redF() green:color.greenF() blue:color.blueF() alpha:color.alphaF()];
+
+    // Unfortunately there seems to be no easy way to set the titlebar text color.
+    // The closest we can do without dubious hacks is set the dark/light mode state based on the brightness of the
+    // background color, which should at least make the text readable even if we can't use the theme's text color.
+    // It's a good idea to set this anyway since it also affects some other UI elements like text shadows (PrismLauncher#3825).
+    if (color.lightnessF() < 0.5) {
+        window.appearance = [NSAppearance appearanceNamed:NSAppearanceNameDarkAqua];
+    } else {
+        window.appearance = [NSAppearance appearanceNamed:NSAppearanceNameAqua];
+    }
+}
+
+void ThemeManager::setTitlebarColorOfAllWindowsOnMac(QColor color)
+{
+    NSArray<NSWindow*>* windows = [NSApp windows];
+    for (NSWindow* window : windows) {
+        setTitlebarColorOnMac((WId)window.contentView, color);
+    }
+
+    // We want to change the titlebar color of newly opened windows as well.
+    // There's no notification for when a new window is opened, but we can set the color when a window switches
+    // from occluded to visible, which also fires on open.
+    NSNotificationCenter* center = [NSNotificationCenter defaultCenter];
+    stopSettingNewWindowColorsOnMac();
+    m_windowTitlebarObserver = [center addObserverForName:NSWindowDidChangeOcclusionStateNotification
+                                                   object:nil
+                                                    queue:[NSOperationQueue mainQueue]
+                                               usingBlock:^(NSNotification* notification) {
+                                                   NSWindow* window = notification.object;
+                                                   setTitlebarColorOnMac((WId)window.contentView, color);
+                                               }];
+}
+
+void ThemeManager::stopSettingNewWindowColorsOnMac()
+{
+    if (m_windowTitlebarObserver) {
+        NSNotificationCenter* center = [NSNotificationCenter defaultCenter];
+        [center removeObserver:m_windowTitlebarObserver];
+        m_windowTitlebarObserver = nil;
+    }
+}