Call updateHighDpiScaling() on screenAdded()

QHighDpiScaling has two init/update functions:
 - initHighDpiScaling(): called once during QGuiApplication construction
 - updateHighDpiScaling(): called whenever (relevant) screen configuration changes

Currently the calls to updateHighDpiScaling() are made from
multiple places including platform code. Simplify by calling
it from two locations:
	- QWindowSystemInterface::handleScreenAdded()
	- QGuiApplicationPrivate::processScreenLogicalDotsPerInchChange()

Replace comment about early calls to qt_defaultDpi with a
test which calls qt_defaultDpiX/Y with no screens attached.
(Looking at the qt_defaultDpiX() implementation, it is unlikely
that there will be a problem as long as updateHighDpiScaling()
is called before QGuiApplication::primaryScreen() starts returning
a non-null value.)

Change-Id: I447db42894617495843a5cb531a1322b000fed62
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

6 files changed, 11 insertions, 18 deletions

diff --git a/src/gui/kernel/qguiapplication.cpp b/src/gui/kernel/qguiapplication.cpp
index 247d9a8d1b..114beb6dde 100644
--- a/src/gui/kernel/qguiapplication.cpp
+++ b/src/gui/kernel/qguiapplication.cpp
@@ -153,8 +153,6 @@ Qt::ApplicationState QGuiApplicationPrivate::applicationState = Qt::ApplicationI
 Qt::HighDpiScaleFactorRoundingPolicy QGuiApplicationPrivate::highDpiScaleFactorRoundingPolicy =
     Qt::HighDpiScaleFactorRoundingPolicy::PassThrough;
 
-bool QGuiApplicationPrivate::highDpiScalingUpdated = false;
-
 QPointer<QWindow> QGuiApplicationPrivate::currentDragWindow;
 
 QList<QGuiApplicationPrivate::TabletPointData> QGuiApplicationPrivate::tabletDevicePoints; // TODO remove
@@ -709,7 +707,6 @@ QGuiApplication::~QGuiApplication()
     QGuiApplicationPrivate::lastCursorPosition = {qreal(qInf()), qreal(qInf())};
     QGuiApplicationPrivate::currentMousePressWindow = QGuiApplicationPrivate::currentMouseWindow = nullptr;
     QGuiApplicationPrivate::applicationState = Qt::ApplicationInactive;
-    QGuiApplicationPrivate::highDpiScalingUpdated = false;
     QGuiApplicationPrivate::currentDragWindow = nullptr;
     QGuiApplicationPrivate::tabletDevicePoints.clear();
 }
@@ -1245,13 +1242,6 @@ static void init_platform(const QString &pluginNamesWithArguments, const QString
         return;
     }
 
-    // Many platforms have created QScreens at this point. Finish initializing
-    // QHighDpiScaling to be prepared for early calls to qt_defaultDpi().
-    if (QGuiApplication::primaryScreen()) {
-        QGuiApplicationPrivate::highDpiScalingUpdated = true;
-        QHighDpiScaling::updateHighDpiScaling();
-    }
-
     // Create the platform theme:
 
     // 1) Fetch the platform name from the environment if present.
@@ -1526,11 +1516,6 @@ void QGuiApplicationPrivate::eventDispatcherReady()
         createPlatformIntegration();
 
     platform_integration->initialize();
-
-    // All platforms should have added screens at this point. Finish
-    // QHighDpiScaling initialization if it has not been done so already.
-    if (!QGuiApplicationPrivate::highDpiScalingUpdated)
-        QHighDpiScaling::updateHighDpiScaling();
 }
 
 void QGuiApplicationPrivate::init()
diff --git a/src/gui/kernel/qguiapplication_p.h b/src/gui/kernel/qguiapplication_p.h
index 57f1b1cc47..39bc5f36a4 100644
--- a/src/gui/kernel/qguiapplication_p.h
+++ b/src/gui/kernel/qguiapplication_p.h
@@ -233,7 +233,6 @@ public:
     static QWindow *currentMousePressWindow;
     static Qt::ApplicationState applicationState;
     static Qt::HighDpiScaleFactorRoundingPolicy highDpiScaleFactorRoundingPolicy;
-    static bool highDpiScalingUpdated;
     static QPointer<QWindow> currentDragWindow;
 
     // TODO remove this: QPointingDevice can store what we need directly
diff --git a/src/gui/kernel/qwindowsysteminterface.cpp b/src/gui/kernel/qwindowsysteminterface.cpp
index dcce42ffe0..58bb0f1cc9 100644
--- a/src/gui/kernel/qwindowsysteminterface.cpp
+++ b/src/gui/kernel/qwindowsysteminterface.cpp
@@ -788,6 +788,7 @@ void QWindowSystemInterface::handleScreenAdded(QPlatformScreen *ps, bool isPrima
         QGuiApplicationPrivate::screen_list.append(screen);
 
     QGuiApplicationPrivate::resetCachedDevicePixelRatio();
+    QHighDpiScaling::updateHighDpiScaling();
 
     emit qGuiApp->screenAdded(screen);
 
diff --git a/src/plugins/platforms/windows/qwindowsscreen.cpp b/src/plugins/platforms/windows/qwindowsscreen.cpp
index 19557bfcd0..2544dd6200 100644
--- a/src/plugins/platforms/windows/qwindowsscreen.cpp
+++ b/src/plugins/platforms/windows/qwindowsscreen.cpp
@@ -641,7 +641,6 @@ bool QWindowsScreenManager::handleScreenChanges()
         if (auto theme = QWindowsTheme::instance()) // QTBUG-85734/Wine
             theme->refreshFonts();
     }
-    QHighDpiScaling::updateHighDpiScaling();
     return true;
 }
 
diff --git a/src/plugins/platforms/xcb/qxcbconnection_screens.cpp b/src/plugins/platforms/xcb/qxcbconnection_screens.cpp
index 2e48826635..867bbef020 100644
--- a/src/plugins/platforms/xcb/qxcbconnection_screens.cpp
+++ b/src/plugins/platforms/xcb/qxcbconnection_screens.cpp
@@ -156,7 +156,6 @@ void QXcbConnection::updateScreens(const xcb_randr_notify_event_t *event)
                     screen = createScreen(virtualDesktop, output, outputInfo.get());
                     qCDebug(lcQpaScreen) << "output" << screen->name() << "is connected and enabled";
                 }
-                QHighDpiScaling::updateHighDpiScaling();
             }
         } else if (screen) {
             if (output.crtc == XCB_NONE && output.mode == XCB_NONE) {
diff --git a/tests/auto/gui/kernel/qhighdpi/tst_qhighdpi.cpp b/tests/auto/gui/kernel/qhighdpi/tst_qhighdpi.cpp
index 31a41eeff6..4082611a0a 100644
--- a/tests/auto/gui/kernel/qhighdpi/tst_qhighdpi.cpp
+++ b/tests/auto/gui/kernel/qhighdpi/tst_qhighdpi.cpp
@@ -314,6 +314,11 @@ void tst_QHighDpi::minimumDpr()
     }
 }
 
+QT_BEGIN_NAMESPACE
+extern int qt_defaultDpiX();
+extern int qt_defaultDpiY();
+QT_END_NAMESPACE
+
 void tst_QHighDpi::noscreens()
 {
     // Create application object with a no-screens configuration (should not crash)
@@ -321,6 +326,11 @@ void tst_QHighDpi::noscreens()
     std::unique_ptr<QGuiApplication> app(createStandardOffscreenApp(noScreens));
 
     QCOMPARE(qApp->devicePixelRatio(), 1);
+
+    // Test calling qt_defaultDpiX/Y: These may be called early during QGuiApplication
+    // initialization, before the platform plugin has created screen objects. They
+    // should then 1) not crash and 2) return some default value.
+    QCOMPARE(qt_defaultDpiX(), qt_defaultDpiY());
 }
 
 void tst_QHighDpi::screenAt_data()