From f061c413de2e3fdc4cc1578f420651a7c45da3e9 Mon Sep 17 00:00:00 2001
From: Friedemann Kleint <Friedemann.Kleint@qt.io>
Date: Thu, 23 Apr 2020 06:32:56 +0200
Subject: [PATCH] Windows: Fix wrong DPI used in font size after changing
 scaling
MIME-Version: 1.0
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QWindowsFontDatabase::defaultVerticalDPI(), which was used
for converting the point sizes was missing an updating
logic for scaling changes. When implementing it, it turned
out that the value obtained from GetDC(0) does not adapt
to scaling changes.

Remove the function and set it from the screen manager directly
to the DPI of the primary screen.

Pick-to: 5.15
Task-number: QTBUG-82267
Change-Id: If05ebc893fe78a9461500aba97f2dc127cdf4406
Reviewed-by: André de la Rocha <andre.rocha@qt.io>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>
---
 .../windows/qwindowsfontdatabasebase.cpp      | 19 ++++++++-----------
 .../windows/qwindowsfontdatabasebase_p.h      |  2 ++
 .../platforms/windows/qwindowsscreen.cpp      |  6 +++++-
 3 files changed, 15 insertions(+), 12 deletions(-)

diff --git a/src/platformsupport/fontdatabases/windows/qwindowsfontdatabasebase.cpp b/src/platformsupport/fontdatabases/windows/qwindowsfontdatabasebase.cpp
index e6dee54d849..568d1463e7f 100644
--- a/src/platformsupport/fontdatabases/windows/qwindowsfontdatabasebase.cpp
+++ b/src/platformsupport/fontdatabases/windows/qwindowsfontdatabasebase.cpp
@@ -624,19 +624,16 @@ void QWindowsFontDatabaseBase::createDirectWriteFactory(IDWriteFactory **factory
 }
 #endif // !defined(QT_NO_DIRECTWRITE)
 
+static int s_defaultVerticalDPI = 96; // Native Pixels
+
 int QWindowsFontDatabaseBase::defaultVerticalDPI()
 {
-    static int vDPI = -1;
-    if (vDPI == -1) {
-        if (HDC defaultDC = GetDC(0)) {
-            vDPI = GetDeviceCaps(defaultDC, LOGPIXELSY);
-            ReleaseDC(0, defaultDC);
-        } else {
-            // FIXME: Resolve now or return 96 and keep unresolved?
-            vDPI = 96;
-        }
-    }
-    return vDPI;
+    return s_defaultVerticalDPI;
+}
+
+void QWindowsFontDatabaseBase::setDefaultVerticalDPI(int d)
+{
+    s_defaultVerticalDPI = d;
 }
 
 LOGFONT QWindowsFontDatabaseBase::fontDefToLOGFONT(const QFontDef &request, const QString &faceName)
diff --git a/src/platformsupport/fontdatabases/windows/qwindowsfontdatabasebase_p.h b/src/platformsupport/fontdatabases/windows/qwindowsfontdatabasebase_p.h
index 5b9db5dede1..55763f7c762 100644
--- a/src/platformsupport/fontdatabases/windows/qwindowsfontdatabasebase_p.h
+++ b/src/platformsupport/fontdatabases/windows/qwindowsfontdatabasebase_p.h
@@ -93,6 +93,8 @@ public:
     QFontEngine *fontEngine(const QByteArray &fontData, qreal pixelSize, QFont::HintingPreference hintingPreference) override;
 
     static int defaultVerticalDPI();
+    static void setDefaultVerticalDPI(int d);
+
     static QSharedPointer<QWindowsFontEngineData> data();
 #if !defined(QT_NO_DIRECTWRITE)
     static void createDirectWriteFactory(IDWriteFactory **factory);
diff --git a/src/plugins/platforms/windows/qwindowsscreen.cpp b/src/plugins/platforms/windows/qwindowsscreen.cpp
index 8850d8cdd42..689624ff2a1 100644
--- a/src/plugins/platforms/windows/qwindowsscreen.cpp
+++ b/src/plugins/platforms/windows/qwindowsscreen.cpp
@@ -51,6 +51,7 @@
 #include <QtGui/qguiapplication.h>
 #include <qpa/qwindowsysteminterface.h>
 #include <private/qhighdpiscaling_p.h>
+#include <private/qwindowsfontdatabasebase_p.h>
 #include <QtGui/qscreen.h>
 
 #include <QtCore/qdebug.h>
@@ -111,8 +112,11 @@ static bool monitorData(HMONITOR hMonitor, QWindowsScreenData *data)
     // EnumDisplayMonitors (as opposed to EnumDisplayDevices) enumerates only
     // virtual desktop screens.
     data->flags |= QWindowsScreenData::VirtualDesktop;
-    if (info.dwFlags & MONITORINFOF_PRIMARY)
+    if (info.dwFlags & MONITORINFOF_PRIMARY) {
         data->flags |= QWindowsScreenData::PrimaryScreen;
+        if ((data->flags & QWindowsScreenData::LockScreen) == 0)
+            QWindowsFontDatabase::setDefaultVerticalDPI(data->dpi.second);
+    }
     return true;
 }