Windows: Decouple screen change monitoring from top level QWindows

The WM_DISPLAYCHANGE message it sent when displays are added, removed,
or update their properties such as the scale/DPI.

We were processing this message as part of QWindowsContext::windowsProc(),
which meant that we would only react to display changes if there was a
QWindow on screen. Just creating a QGuiApplication was insufficient to
pick up changes to screens after startup.

In addition, despite being documented to post messages to child windows,
WM_DISPLAYCHANGE only ends up in top level windows. Presumably it's the
top level window's responsibility to post the message to child windows.
As a result, if a QWindow was a native child window of a foreign window,
such as in audio plugins being hosted in a DAW, we would again fail to
pick up display changes.

We solve both these cases by decoupling the WM_DISPLAYCHANGE handling
from QWindowsContext::windowsProc(), by creating a dedicated window
for listening to WM_DISPLAYCHANGE. This is similar to how we already
handle tray icons, power notifications, clipboard, etc -- the only
difference being that since purely HWND_MESSAGE windows do not
receive WM_DISPLAYCHANGE it's an actual invisible WS_TILED window.

This also lets us remove the workaround for QTBUG-79248, which was
doing screen updates in response to WM_DPICHANGED when detecting
that there were no QWindows.

Task-number: QTBUG-103383
Task-number: QTBUG-79248
Fixes: QTBUG-102343
Change-Id: I905d8253069ec339b193edf05c052d21361ca3e9
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
(cherry picked from commit fa0b2ef81c0d22f4038235871fbc1abda55887d1)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/plugins/platforms/windows/qtwindowsglobal.h

@@ -140,8 +140,7 @@ enum WindowsEventType // Simplify event types
     InputMethodRequest = InputMethodEventFlag + 6,
     ThemeChanged = ThemingEventFlag + 1,
     CompositionSettingsChanged = ThemingEventFlag + 2,
-    DisplayChangedEvent = 437,
-    SettingChangedEvent = DisplayChangedEvent + 1,
+    SettingChangedEvent = 438,
     ScrollEvent = GenericEventFlag + 1,
     ContextMenu = 123,
     GestureEvent = 124,
@@ -258,8 +257,6 @@ inline QtWindows::WindowsEventType windowsEventType(UINT message, WPARAM wParamI
     // http://msdn.microsoft.com/en-us/library/ms695534(v=vs.85).aspx
     case WM_SETTINGCHANGE:
         return QtWindows::SettingChangedEvent;
-    case WM_DISPLAYCHANGE:
-        return QtWindows::DisplayChangedEvent;
     case WM_THEMECHANGED:
     case WM_SYSCOLORCHANGE: // Handle color change as theme change (QTBUG-34170).
     case WM_DWMCOLORIZATIONCOLORCHANGED:

src/plugins/platforms/windows/qwindowscontext.cpp

@@ -71,6 +71,7 @@ Q_LOGGING_CATEGORY(lcQpaTablet, "qt.qpa.input.tablet")
 Q_LOGGING_CATEGORY(lcQpaAccessibility, "qt.qpa.accessibility")
 Q_LOGGING_CATEGORY(lcQpaUiAutomation, "qt.qpa.uiautomation")
 Q_LOGGING_CATEGORY(lcQpaTrayIcon, "qt.qpa.trayicon")
+Q_LOGGING_CATEGORY(lcQpaScreen, "qt.qpa.screen")
 
 int QWindowsContext::verbose = 0;
 
@@ -1091,12 +1092,6 @@ bool QWindowsContext::windowsProc(HWND hwnd, UINT message,
 #else
         return false;
 #endif
-    case QtWindows::DisplayChangedEvent:
-        if (QWindowsTheme *t = QWindowsTheme::instance())
-            t->displayChanged();
-        QWindowsWindow::displayChanged();
-        d->m_screenManager.handleScreenChanges();
-        return false;
     case QtWindows::SettingChangedEvent: {
         QWindowsWindow::settingsChanged();
         // Only refresh the window theme if the user changes the personalize settings.

src/plugins/platforms/windows/qwindowscontext.h

@@ -31,6 +31,7 @@ Q_DECLARE_LOGGING_CATEGORY(lcQpaTablet)
 Q_DECLARE_LOGGING_CATEGORY(lcQpaAccessibility)
 Q_DECLARE_LOGGING_CATEGORY(lcQpaUiAutomation)
 Q_DECLARE_LOGGING_CATEGORY(lcQpaTrayIcon)
+Q_DECLARE_LOGGING_CATEGORY(lcQpaScreen)
 
 class QWindow;
 class QPlatformScreen;

src/plugins/platforms/windows/qwindowsintegration.cpp

@@ -264,7 +264,7 @@ QWindowsIntegration::QWindowsIntegration(const QStringList &paramList) :
 #if QT_CONFIG(clipboard)
     d->m_clipboard.registerViewer();
 #endif
-    d->m_context.screenManager().handleScreenChanges();
+    d->m_context.screenManager().initialize();
     d->m_context.setDetectAltGrModifier((d->m_options & DetectAltGrModifier) != 0);
 }
 

src/plugins/platforms/windows/qwindowsscreen.cpp

@@ -513,8 +513,45 @@ QPlatformScreen::SubpixelAntialiasingType QWindowsScreen::subpixelAntialiasingTy
     \internal
 */
 
+extern "C" LRESULT QT_WIN_CALLBACK qDisplayChangeObserverWndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
+{
+    if (message == WM_DISPLAYCHANGE) {
+        qCDebug(lcQpaScreen) << "Handling WM_DISPLAYCHANGE";
+        if (QWindowsTheme *t = QWindowsTheme::instance())
+            t->displayChanged();
+        QWindowsWindow::displayChanged();
+        QWindowsContext::instance()->screenManager().handleScreenChanges();
+    }
+
+    return DefWindowProc(hwnd, message, wParam, lParam);
+}
+
 QWindowsScreenManager::QWindowsScreenManager() = default;
 
+void QWindowsScreenManager::initialize()
+{
+    qCDebug(lcQpaScreen) << "Initializing screen manager";
+
+    auto className = QWindowsContext::instance()->registerWindowClass(
+        QWindowsContext::classNamePrefix() + QLatin1String("ScreenChangeObserverWindow"),
+        qDisplayChangeObserverWndProc);
+
+    // HWND_MESSAGE windows do not get WM_DISPLAYCHANGE, so we need to create
+    // a real top level window that we never show.
+    m_displayChangeObserver = CreateWindowEx(0, reinterpret_cast<LPCWSTR>(className.utf16()),
+        nullptr, WS_TILED, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT,
+        nullptr, nullptr, GetModuleHandle(nullptr), nullptr);
+    Q_ASSERT(m_displayChangeObserver);
+
+    qCDebug(lcQpaScreen) << "Created display change observer" << m_displayChangeObserver;
+
+    handleScreenChanges();
+}
+
+QWindowsScreenManager::~QWindowsScreenManager()
+{
+    DestroyWindow(m_displayChangeObserver);
+}
 
 bool QWindowsScreenManager::isSingleScreen()
 {

src/plugins/platforms/windows/qwindowsscreen.h

@@ -96,6 +96,8 @@ public:
     using WindowsScreenList = QList<QWindowsScreen *>;
 
     QWindowsScreenManager();
+    void initialize();
+    ~QWindowsScreenManager();
 
     void clearScreens();
 
@@ -110,6 +112,7 @@ public:
 private:
     void removeScreen(int index);
 
+    HWND m_displayChangeObserver = nullptr;
     WindowsScreenList m_screens;
 };
 

src/plugins/platforms/windows/qwindowssystemtrayicon.cpp

@@ -94,9 +94,6 @@ static int indexOfHwnd(HWND hwnd)
 extern "C" LRESULT QT_WIN_CALLBACK qWindowsTrayIconWndProc(HWND hwnd, UINT message,
                                                            WPARAM wParam, LPARAM lParam)
 {
-    // QTBUG-79248: Trigger screen update if there are no other windows.
-    if (message == WM_DPICHANGED && QGuiApplication::topLevelWindows().isEmpty())
-        QWindowsContext::instance()->screenManager().handleScreenChanges();
     if (message == MYWM_TASKBARCREATED || message == MYWM_NOTIFYICON
         || message == WM_INITMENU || message == WM_INITMENUPOPUP
         || message == WM_CLOSE || message == WM_COMMAND) {