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, 1 insertions, 0 deletions

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);