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
Pick-to: 6.4
Change-Id: I905d8253069ec339b193edf05c052d21361ca3e9
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

1 files changed, 1 insertions, 6 deletions

diff --git a/src/plugins/platforms/windows/qwindowscontext.cpp b/src/plugins/platforms/windows/qwindowscontext.cpp
index 68d2e5d98d..9397c0213e 100644
--- a/src/plugins/platforms/windows/qwindowscontext.cpp
+++ b/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.