From ade0368f100dddcad2a0ced78e285cc3d561ce58 Mon Sep 17 00:00:00 2001
From: Moritz Bunkus <mo@bunkus.online>
Date: Wed, 20 Oct 2021 21:43:36 +0200
Subject: [PATCH] GUI: Windows: use pass through high DPI scaling factor
 rounding policy

Means fractional scaling factors such as Windows' 175% will no longer
use 200% (integer rounding) etc.

This is the default on Qt 6 anyway.

Implements #3043.
---
 NEWS.md                                        |  5 +++++
 src/mkvtoolnix-gui/mkvtoolnix_gui.cpp          | 10 ++++++++--
 src/mkvtoolnix-gui/util/system_information.cpp |  2 +-
 3 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/NEWS.md b/NEWS.md
index 78afedef4..a7428d942 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -4,6 +4,11 @@
 
 * mkvmerge: AC-3 parser: E-AC-3 with BSID values > 10 and ≤ 15 are recognized
   now, too. Implements #3211.
+* MKVToolNix GUI: only on Windows: when compiled with Qt ≥ 5.14 and < 6 (which
+  is the case for the officially provided binaries) fractional screen scaling
+  will be enabled. This means that the GUI will be scaled appropriately if
+  Windows is set to 125%, 150% or 175% instead of being too small or too
+  large. Implements #3043.
 
 ## Bug fixes
 
diff --git a/src/mkvtoolnix-gui/mkvtoolnix_gui.cpp b/src/mkvtoolnix-gui/mkvtoolnix_gui.cpp
index 8f9c50ab1..d72a0ec50 100644
--- a/src/mkvtoolnix-gui/mkvtoolnix_gui.cpp
+++ b/src/mkvtoolnix-gui/mkvtoolnix_gui.cpp
@@ -30,8 +30,14 @@ enableOrDisableHighDPIScaling() {
   auto reg = Util::Settings::registry();
   reg->beginGroup(s_grpSettings);
 #if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
-  if (!reg->value(s_valUiDisableHighDPIScaling).toBool())
-    QApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
+  if (reg->value(s_valUiDisableHighDPIScaling).toBool())
+    return;
+
+  QApplication::setAttribute(Qt::AA_EnableHighDpiScaling, true);
+  QApplication::setAttribute(Qt::AA_UseHighDpiPixmaps,    true);
+# if QT_VERSION >= QT_VERSION_CHECK(5, 14, 0)
+  QApplication::setHighDpiScaleFactorRoundingPolicy(Qt::HighDpiScaleFactorRoundingPolicy::PassThrough);
+# endif
 #endif
 }
 
diff --git a/src/mkvtoolnix-gui/util/system_information.cpp b/src/mkvtoolnix-gui/util/system_information.cpp
index a2749c63e..afbc01eba 100644
--- a/src/mkvtoolnix-gui/util/system_information.cpp
+++ b/src/mkvtoolnix-gui/util/system_information.cpp
@@ -133,7 +133,7 @@ void
 gatherEnvironmentVariables(QStringList &info) {
   info << Q("") << Q("## Environment variables") << Q("");
 
-  auto keys = QStringList{} << Q("QT_AUTO_SCREEN_SCALE_FACTOR") << Q("QT_SCALE_FACTOR") << Q("QT_SCREEN_SCALE_FACTORS") << Q("QT_DEVICE_PIXEL_RATIO")
+  auto keys = QStringList{} << Q("QT_AUTO_SCREEN_SCALE_FACTOR") << Q("QT_SCALE_FACTOR") << Q("QT_SCREEN_SCALE_FACTORS") << Q("QT_DEVICE_PIXEL_RATIO") << Q("QT_SCALE_FACTOR_ROUNDING_POLICY")
                             << Q("MTX_LOGGER")                  << Q("MTX_DEBUG")       << Q("MKVTOOLNIX_DEBUG")        << Q("MKVMERGE_DEBUG")
                             << Q("LC_ALL")                      << Q("LC_MESSAGES")     << Q("LC_CTYPE")                << Q("LANG") << Q("LANGUAGE");
   keys.sort();