From 54c2c9ac4e897a5a1ebbdbfa15d94fc452b72ac5 Mon Sep 17 00:00:00 2001
From: Jocelyn Turcotte <jturcotte@woboq.com>
Date: Mon, 11 Jan 2016 15:27:13 +0100
Subject: [PATCH] Windows: Fix HiDPI #3414

Use QT_DEVICE_PIXEL_RATIO=auto on Qt<=5.5 to enable automatic
scale factor settings on Windows. Also move the existing
Qt::AA_EnableHighDpiScaling logic to use the equivalent
QT_AUTO_SCREEN_SCALE_FACTOR=1 environment variable just to
keep the 5.5 and >=5.6 code at the same place.
---
 src/gui/application.cpp |  6 +-----
 src/gui/main.cpp        | 23 +++++++++++++++++++++++
 2 files changed, 24 insertions(+), 5 deletions(-)

diff --git a/src/gui/application.cpp b/src/gui/application.cpp
index 301856e89..af1764b8e 100644
--- a/src/gui/application.cpp
+++ b/src/gui/application.cpp
@@ -113,11 +113,7 @@ Application::Application(int &argc, char **argv) :
 #if QT_VERSION > QT_VERSION_CHECK(5, 0, 0)
     setAttribute(Qt::AA_UseHighDpiPixmaps, true);
 #endif
-#if QT_VERSION > QT_VERSION_CHECK(5, 6, 0)
-    // this may slightly break some styles until they
-    // get fixed, but will make dialogs readable.
-    setAttribute(Qt::AA_EnableHighDpiScaling, true);
-#endif
+
     parseOptions(arguments());
     //no need to waste time;
     if ( _helpOnly || _versionOnly ) return;
diff --git a/src/gui/main.cpp b/src/gui/main.cpp
index 83c38063f..28a6858f6 100644
--- a/src/gui/main.cpp
+++ b/src/gui/main.cpp
@@ -47,11 +47,34 @@ int main(int argc, char **argv)
 {
     Q_INIT_RESOURCE(client);
 
+#ifdef Q_OS_WIN
+    // If the font size ratio is set on Windows, we need to
+    // enable the auto pixelRatio in Qt since we don't
+    // want to use sizes relative to the font size everywhere.
+    // This is automatic on OS X, but opt-in on Windows
+    // https://doc-snapshots.qt.io/qt5-5.6/highdpi.html#qt-support
+#if QT_VERSION < QT_VERSION_CHECK(5, 6, 0)
+    qputenv("QT_DEVICE_PIXEL_RATIO", "auto");
+#else
+    qputenv("QT_AUTO_SCREEN_SCALE_FACTOR", "1");
+#endif
+#endif // Q_OS_WIN
+
 #ifdef Q_OS_MAC
     Mac::CocoaInitializer cocoaInit; // RIIA
 #endif
     OCC::Application app(argc, argv);
 
+#ifdef Q_OS_WIN
+    // The Windows style still has pixelated elements with Qt 5.6,
+    // it's recommended to use the Fusion style in this case, even
+    // though it looks slightly less native. Check here after the
+    // QApplication was constructed, but before any QWidget is
+    // constructed.
+    if (app.devicePixelRatio() > 1)
+        QApplication::setStyle(QStringLiteral("fusion"));
+#endif // Q_OS_WIN
+
 #ifndef Q_OS_WIN
     signal(SIGPIPE, SIG_IGN);
 #endif
-- 
2.30.2