Core: Add a setting for HighDpiScaleFactorRoundingPolicy

In Qt 6, the API and environment variables intended to influence the
HighDPI behavior changed quite a bit. Until recently, (see "Amends"
change), Qt Creator still supported the Qt 5 options instead of the Qt 6
options.

With Qt 6, HighDpi support is always enabled. Just the DPI rounding
policy should be set by user or application.

This change adds a ComboBox in the General settings, which allows to
select a DPI rounding policy:
  https://doc.qt.io/qt-6/qt.html#HighDpiScaleFactorRoundingPolicy-enum

The default setting is "PassThrough", which is the default policy for Qt
6, and which is also the default in Qt Creator since version 11.0.2.

Amends: d1deeb2c0d

Fixes: QTCREATORBUG-29501
Change-Id: Ib19b40cb3945cecc71b7c10f42fa2b06b3cc6153
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/app/main.cpp

@@ -285,11 +285,11 @@ static void setHighDpiEnvironmentVariable()
 
     std::unique_ptr<Utils::QtcSettings> settings(createUserSettings());
 
-    const bool defaultValue = Utils::HostOsInfo::isWindowsHost();
-    const bool enableHighDpiScaling = settings->value("Core/EnableHighDpiScaling", defaultValue).toBool();
-    const auto policy = enableHighDpiScaling ? Qt::HighDpiScaleFactorRoundingPolicy::PassThrough
-                                             : Qt::HighDpiScaleFactorRoundingPolicy::Floor;
-    QGuiApplication::setHighDpiScaleFactorRoundingPolicy(policy);
+    using Policy = Qt::HighDpiScaleFactorRoundingPolicy;
+    const Policy defaultPolicy = Utils::HostOsInfo::defaultHighDpiScaleFactorRoundingPolicy();
+    const Policy userPolicy = settings->value("Core/HighDpiScaleFactorRoundingPolicy",
+                                              int(defaultPolicy)).value<Policy>();
+    QGuiApplication::setHighDpiScaleFactorRoundingPolicy(userPolicy);
 }
 
 void setPixmapCacheLimit()
@@ -522,8 +522,6 @@ int main(int argc, char **argv)
     }
 
     qputenv("QSG_RHI_BACKEND", "opengl");
-    QGuiApplication::setHighDpiScaleFactorRoundingPolicy(
-                Qt::HighDpiScaleFactorRoundingPolicy::Round);
 
     if (qEnvironmentVariableIsSet("QTCREATOR_DISABLE_NATIVE_MENUBAR")
             || qgetenv("XDG_CURRENT_DESKTOP").startsWith("Unity")) {

src/libs/utils/hostosinfo.h

@@ -90,6 +90,12 @@ public:
 
     static const FilePath &root();
 
+    static constexpr Qt::HighDpiScaleFactorRoundingPolicy
+    defaultHighDpiScaleFactorRoundingPolicy() {
+        return isMacHost() ? Qt::HighDpiScaleFactorRoundingPolicy::Unset
+                           : Qt::HighDpiScaleFactorRoundingPolicy::PassThrough;
+    }
+
 private:
     static Qt::CaseSensitivity m_overrideFileNameCaseSensitivity;
     static bool m_useOverrideFileNameCaseSensitivity;

src/plugins/coreplugin/generalsettings.cpp

@@ -37,7 +37,7 @@ using namespace Layouting;
 
 namespace Core::Internal {
 
-const char settingsKeyDPI[] = "Core/EnableHighDpiScaling";
+const char settingsKeyDpiPolicy[] = "Core/HighDpiScaleFactorRoundingPolicy";
 const char settingsKeyCodecForLocale[] = "General/OverrideCodecForLocale";
 const char settingsKeyToolbarStyle[] = "General/ToolbarStyle";
 
@@ -89,6 +89,7 @@ public:
     static QByteArray codecForLocale();
     static void setCodecForLocale(const QByteArray&);
     void fillToolbarSyleBox() const;
+    static void setDpiPolicy(Qt::HighDpiScaleFactorRoundingPolicy policy);
 
     QComboBox *m_languageBox;
     QComboBox *m_codecBox;
@@ -96,6 +97,7 @@ public:
     ThemeChooser *m_themeChooser;
     QPushButton *m_resetWarningsButton;
     QComboBox *m_toolbarStyleBox;
+    QComboBox *m_policyComboBox = nullptr;
 };
 
 GeneralSettingsWidget::GeneralSettingsWidget()
@@ -134,16 +136,23 @@ GeneralSettingsWidget::GeneralSettingsWidget()
     form.addRow({Tr::tr("Language:"), m_languageBox, st});
 
     if (!Utils::HostOsInfo::isMacHost()) {
-        auto dpiCheckbox = new QCheckBox(Tr::tr("Enable high DPI scaling"));
-        form.addRow({empty, dpiCheckbox});
-        const bool defaultValue = Utils::HostOsInfo::isWindowsHost();
-        dpiCheckbox->setChecked(ICore::settings()->value(settingsKeyDPI, defaultValue).toBool());
-        connect(dpiCheckbox, &QCheckBox::toggled, this, [defaultValue](bool checked) {
-            ICore::settings()->setValueWithDefault(settingsKeyDPI, checked, defaultValue);
-            QMessageBox::information(ICore::dialogParent(),
-                                     Tr::tr("Restart Required"),
-                                     Tr::tr("The high DPI settings will take effect after restart."));
-        });
+        using Policy = Qt::HighDpiScaleFactorRoundingPolicy;
+        m_policyComboBox = new QComboBox;
+        m_policyComboBox->addItem(Tr::tr("Round up for .5 and above"), int(Policy::Round));
+        m_policyComboBox->addItem(Tr::tr("Always round up"), int(Policy::Ceil));
+        m_policyComboBox->addItem(Tr::tr("Always round down"), int(Policy::Floor));
+        m_policyComboBox->addItem(Tr::tr("Round up for .75 and above"),
+                                int(Policy::RoundPreferFloor));
+        m_policyComboBox->addItem(Tr::tr("Don't round"), int(Policy::PassThrough));
+        m_policyComboBox->setSizeAdjustPolicy(QComboBox::AdjustToContents);
+
+        const Policy userPolicy =
+            ICore::settings()->value(settingsKeyDpiPolicy,
+                                     int(HostOsInfo::defaultHighDpiScaleFactorRoundingPolicy()))
+                                      .value<Policy>();
+        m_policyComboBox->setCurrentIndex(m_policyComboBox->findData(int(userPolicy)));
+
+        form.addRow({Tr::tr("DPI Rounding Policy:"), m_policyComboBox, st});
     }
 
     form.addRow({empty, generalSettings().showShortcutsInContextMenus});
@@ -214,6 +223,11 @@ void GeneralSettingsWidget::apply()
 
     int currentIndex = m_languageBox->currentIndex();
     setLanguage(m_languageBox->itemData(currentIndex, Qt::UserRole).toString());
+    if (m_policyComboBox) {
+        const Qt::HighDpiScaleFactorRoundingPolicy selectedPolicy =
+            m_policyComboBox->currentData().value<Qt::HighDpiScaleFactorRoundingPolicy>();
+        setDpiPolicy(selectedPolicy);
+    }
     currentIndex = m_codecBox->currentIndex();
     setCodecForLocale(m_codecBox->itemText(currentIndex).toLocal8Bit());
     // Apply the new base color if accepted
@@ -317,6 +331,21 @@ void GeneralSettingsWidget::fillToolbarSyleBox() const
     m_toolbarStyleBox->setCurrentIndex(curId);
 }
 
+void GeneralSettingsWidget::setDpiPolicy(Qt::HighDpiScaleFactorRoundingPolicy policy)
+{
+    QtcSettings *settings = ICore::settings();
+    const Qt::HighDpiScaleFactorRoundingPolicy previousPolicy =
+        settings->value(settingsKeyDpiPolicy).value<Qt::HighDpiScaleFactorRoundingPolicy>();
+    if (policy != previousPolicy) {
+        RestartDialog dialog(ICore::dialogParent(),
+                             Tr::tr("The DPI rounding policy change will take effect after "
+                                    "restart."));
+        dialog.exec();
+    }
+    settings->setValueWithDefault(settingsKeyDpiPolicy, int(policy),
+                                  int(HostOsInfo::defaultHighDpiScaleFactorRoundingPolicy()));
+}
+
 void GeneralSettings::applyToolbarStyleFromSettings()
 {
     StyleHelper::setToolbarStyle(toolbarStylefromSettings());