Enable RoundPreferFloor HighDpi support on Windows.

Add an experimental setting for exact HighDPI on Windows.

Telegram/SourceFiles/core/launcher.cpp

@@ -9,6 +9,7 @@ https://github.com/telegramdesktop/tdesktop/blob/master/LEGAL
 
 #include "platform/platform_launcher.h"
 #include "platform/platform_specific.h"
+#include "base/options.h"
 #include "base/platform/base_platform_info.h"
 #include "base/platform/base_platform_file_utilities.h"
 #include "ui/main_queue_processor.h"
@@ -272,8 +273,18 @@ bool CheckPortableVersionFolder() {
 	return true;
 }
 
+base::options::toggle OptionFractionalScalingEnabled({
+	.id = kOptionFractionalScalingEnabled,
+	.name = "Enable precise High DPI scaling",
+	.description = "Follow system interface scale settings exactly.",
+	.scope = base::options::windows,
+	.restartRequired = true,
+});
+
 } // namespace
 
+const char kOptionFractionalScalingEnabled[] = "fractional-scaling-enabled";
+
 std::unique_ptr<Launcher> Launcher::Create(int argc, char *argv[]) {
 	return std::make_unique<Platform::Launcher>(argc, argv);
 }
@@ -294,9 +305,6 @@ void Launcher::init() {
 	initQtMessageLogging();
 
 	QApplication::setApplicationName(u"TelegramDesktop"_q);
-	QApplication::setAttribute(Qt::AA_DisableHighDpiScaling, true);
-	QApplication::setHighDpiScaleFactorRoundingPolicy(
-		Qt::HighDpiScaleFactorRoundingPolicy::Floor);
 
 #if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
 	// fallback session management is useless for tdesktop since it doesn't have
@@ -311,6 +319,17 @@ void Launcher::init() {
 	initHook();
 }
 
+void Launcher::initHighDpi() {
+	QApplication::setAttribute(Qt::AA_EnableHighDpiScaling, true);
+	if (OptionFractionalScalingEnabled.value()) {
+		QApplication::setHighDpiScaleFactorRoundingPolicy(
+			Qt::HighDpiScaleFactorRoundingPolicy::PassThrough);
+	} else {
+		QApplication::setHighDpiScaleFactorRoundingPolicy(
+			Qt::HighDpiScaleFactorRoundingPolicy::RoundPreferFloor);
+	}
+}
+
 int Launcher::exec() {
 	init();
 
@@ -324,6 +343,9 @@ int Launcher::exec() {
 	Logs::start(this);
 	base::options::init(cWorkingDir() + "tdata/experimental_options.json");
 
+	// Must be called after options are inited.
+	initHighDpi();
+
 	if (Logs::DebugEnabled()) {
 		const auto openalLogPath = QDir::toNativeSeparators(
 			cWorkingDir() + u"DebugLogs/last_openal_log.txt"_q);

Telegram/SourceFiles/core/launcher.h

@@ -11,6 +11,8 @@ https://github.com/telegramdesktop/tdesktop/blob/master/LEGAL
 
 namespace Core {
 
+extern const char kOptionFractionalScalingEnabled[];
+
 class Launcher {
 public:
 	Launcher(int argc, char *argv[]);
@@ -52,6 +54,7 @@ private:
 	void init();
 	virtual void initHook() {
 	}
+	virtual void initHighDpi();
 
 	virtual bool launchUpdater(UpdaterLaunch action) = 0;
 

Telegram/SourceFiles/core/sandbox.cpp

@@ -226,7 +226,12 @@ void Sandbox::setupScreenScale() {
 	logEnv("QT_USE_PHYSICAL_DPI");
 	logEnv("QT_FONT_DPI");
 
-	const auto useRatio = std::clamp(qCeil(ratio), 1, 3);
+	// Like Qt::HighDpiScaleFactorRoundingPolicy::RoundPreferFloor.
+	// Round up for .75 and higher. This favors "small UI" over "large UI".
+	const auto roundedRatio = ((ratio - qFloor(ratio)) < 0.75)
+		? qFloor(ratio)
+		: qCeil(ratio);
+	const auto useRatio = std::clamp(roundedRatio, 1, 3);
 	style::SetDevicePixelRatio(useRatio);
 
 	const auto screen = Sandbox::primaryScreen();

Telegram/SourceFiles/platform/mac/launcher_mac.h

@@ -17,6 +17,7 @@ public:
 
 private:
 	void initHook() override;
+	void initHighDpi() override;
 
 	bool launchUpdater(UpdaterLaunch action) override;
 

Telegram/SourceFiles/platform/mac/launcher_mac.mm

@@ -24,12 +24,14 @@ Launcher::Launcher(int argc, char *argv[])
 }
 
 void Launcher::initHook() {
-	// macOS Retina display support is working fine, others are not.
-	QCoreApplication::setAttribute(Qt::AA_DisableHighDpiScaling, false);
-
 	base::RegisterBundledResources(u"Telegram.rcc"_q);
 }
 
+void Launcher::initHighDpi() {
+	// macOS Retina display support is working fine.
+	QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling, true);
+}
+
 bool Launcher::launchUpdater(UpdaterLaunch action) {
 	if (cExeName().isEmpty()) {
 		return false;

Telegram/SourceFiles/settings/settings_experimental.cpp

@@ -16,6 +16,7 @@ https://github.com/telegramdesktop/tdesktop/blob/master/LEGAL
 #include "ui/chat/chat_style_radius.h"
 #include "base/options.h"
 #include "core/application.h"
+#include "core/launcher.h"
 #include "platform/platform_specific.h"
 #include "chat_helpers/tabbed_panel.h"
 #include "dialogs/dialogs_inner_widget.h"
@@ -137,6 +138,7 @@ void SetupExperimental(
 	};
 
 	addToggle(ChatHelpers::kOptionTabbedPanelShowOnClick);
+	addToggle(Core::kOptionFractionalScalingEnabled);
 	addToggle(Window::kOptionViewProfileInChatsListContextMenu);
 	addToggle(Dialogs::kOptionCtrlClickChatNewWindow);
 	addToggle(Ui::GL::kOptionAllowLinuxNvidiaOpenGL);