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>

1 files changed, 10 insertions, 0 deletions

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()