Fix support for screen fractional scaling with Wayland

The display was wrong when a backing store is in use (which is the case
with Wayland). To fix this in GuiWorkArea::Private::resetScreen(), the
pixelRatio is now stored as a double instead of an int.

Concerning support for QT_SCALE_FACTOR, the existing code was wrong
because this value is already taken into account in devicePixelRatioF
for Qt > 5.6 (no fractional scaling support before that). The
situation is as follows:

                   Qt < 5.6    5.6 <= Qt < 6  Qt 6
devicePixelRatio   int(ratio)  int(ratio)     ratio
devicePixelRatioF  N/A         ratio          ratio

So it is only between Qt 5.6 and Qt 6 that devicePixelRatioF() has to
be used instead of devicePixelRatio().
QGuiApplication::devicePixelRatio() does not have a 'F' version, it
always returns the real thing.

Fixes ticket #13039.

src/LyX.cpp

@@ -50,7 +50,6 @@
 #include "frontends/Application.h"
 
 #include "support/ConsoleApplication.h"
-#include "support/convert.h"
 #include "support/lassert.h"
 #include "support/debug.h"
 #include "support/environment.h"
@@ -121,14 +120,6 @@ RunMode run_mode = PREFERRED;
 OverwriteFiles force_overwrite = UNSPECIFIED;
 
 
-// Scale the GUI by this factor. This works whether we have a HiDpi screen
-// or not and scales everything, also fonts. Can only be changed by setting
-// the QT_SCALE_FACTOR environment variable before launching LyX and only
-// works properly with Qt 5.6 or higher.
-
-double qt_scale_factor = 1.0;
-
-
 namespace {
 
 // Filled with the command line arguments "foo" of "-sysdir foo" or
@@ -318,17 +309,6 @@ int LyX::exec(int & argc, char * argv[])
 	// we need to parse for "-dbg" and "-help"
 	easyParse(argc, argv);
 
-#if QT_VERSION >= 0x050600
-	// Check whether Qt will scale all GUI elements and accordingly
-	// set the scale factor so that to avoid blurred images and text
-	char const * const scale_factor = getenv("QT_SCALE_FACTOR");
-	if (scale_factor) {
-		qt_scale_factor = convert<double>(scale_factor);
-		if (qt_scale_factor < 1.0)
-			qt_scale_factor = 1.0;
-	}
-#endif
-
 	try {
 		init_package(os::utf8_argv(0), cl_system_support, cl_user_support);
 	} catch (ExceptionMessage const & message) {

src/LyX.h

@@ -56,7 +56,6 @@ extern bool verbose;
 extern bool ignore_missing_glyphs;
 extern RunMode run_mode;
 extern OverwriteFiles force_overwrite;
-extern double qt_scale_factor;
 
 namespace frontend {
 class Application;

src/frontends/qt/GuiApplication.cpp

@@ -1247,7 +1247,7 @@ GuiApplication * theGuiApp()
 
 double GuiApplication::pixelRatio() const
 {
-	return qt_scale_factor * devicePixelRatio();
+	return devicePixelRatio();
 }
 
 
@@ -2669,7 +2669,7 @@ QPixmap GuiApplication::getScaledPixmap(QString imagedir, QString name) const
 	qreal dpr = 1.0;
 	// Consider device/pixel ratio (HiDPI)
 	if (currentView())
-		dpr = currentView()->devicePixelRatio();
+		dpr = currentView()->pixelRatio();
 	// We render SVG directly for HiDPI scalability
 	QPixmap pm = getPixmap(imagedir, name, "svgz,png");
 	FileName fname = imageLibFileSearch(imagedir, name, "svgz,png");

src/frontends/qt/GuiView.cpp

@@ -260,7 +260,11 @@ private:
 
 	/// Current ratio between physical pixels and device-independent pixels
 	double pixelRatio() const {
-		return qt_scale_factor * devicePixelRatio();
+#if (QT_VERSION >= QT_VERSION_CHECK(5, 6, 0) && QT_VERSION < QT_VERSION_CHECK(6, 0, 0))
+		return devicePixelRatioF();
+#else
+		return devicePixelRatio();
+#endif
 	}
 
 	qreal fontSize() const {
@@ -1844,7 +1848,11 @@ void GuiView::resetCommandExecute()
 
 double GuiView::pixelRatio() const
 {
-	return qt_scale_factor * devicePixelRatio();
+#if (QT_VERSION >= QT_VERSION_CHECK(5, 6, 0) && QT_VERSION < QT_VERSION_CHECK(6, 0, 0))
+	return devicePixelRatioF();
+#else
+	return devicePixelRatio();
+#endif
 }
 
 

src/frontends/qt/GuiWorkArea.cpp

@@ -179,7 +179,11 @@ GuiWorkArea::GuiWorkArea(Buffer & buffer, GuiView & gv)
 
 double GuiWorkArea::pixelRatio() const
 {
-	return qt_scale_factor * devicePixelRatio();
+#if (QT_VERSION >= QT_VERSION_CHECK(5, 6, 0) && QT_VERSION < QT_VERSION_CHECK(6, 0, 0))
+	return devicePixelRatioF();
+#else
+	return devicePixelRatio();
+#endif
 }
 
 
@@ -1264,9 +1268,9 @@ void GuiWorkArea::Private::paintPreeditText(GuiPainter & pain)
 void GuiWorkArea::Private::resetScreen()
 {
 	if (use_backingstore_) {
-		int const pr = p->pixelRatio();
-		screen_ = QImage(pr * p->viewport()->width(),
-		                 pr * p->viewport()->height(),
+		double const pr = p->pixelRatio();
+		screen_ = QImage(int(pr * p->viewport()->width()),
+		                 int(pr * p->viewport()->height()),
 		                 QImage::Format_ARGB32_Premultiplied);
 		screen_.setDevicePixelRatio(pr);
 	}