Fixing fractional OS scaling support in Wayland

Generally, the problem stems from the fact that Wayland reports its
scaling factor as an integer, which then Qt just takes for granted until
it somehow manages to notice, when initializing a(n OpenGL-enabled)
widget, that the factor is actually not integral, and then we can query
it from the widget. QScreen still remains with this old wrong factor, so
this factor shouldn't be queried from it.

Fixes #4089.

src/StelMainView.cpp

@@ -395,17 +395,14 @@ class StelRootItem : public QGraphicsObject
 		//a sanity check
 		Q_ASSERT(mainView->glContext() == QOpenGLContext::currentContext());
 
-		{
-			static int oldViewportWidth, oldViewportHeight;
-			GLint viewport[4] = {};
-			GL(QOpenGLContext::currentContext()->functions()->glGetIntegerv(GL_VIEWPORT, viewport));
-			if(viewport[2] != oldViewportWidth || viewport[3] != oldViewportHeight)
-			{
-				qDebug() << "Early painting: OpenGL viewport size: " << viewport[2] << "x" << viewport[3];
-				oldViewportWidth = viewport[2];
-				oldViewportHeight = viewport[3];
-			}
-		}
+		StelApp& app = StelApp::getInstance();
+
+		// This can change even on the screen even without actual system settings change.
+		// E.g. in KWin 6.1.5 with Wayland backend, if we set 150% scale in System Settings,
+		// the app first gets device pixel ratio of 200%, then the widgets are rescaled to 150%
+		// while the screen still remains at 200%. This is ugly, and shouldn't behave like this,
+		// but the following call seems to be enough to get things working right.
+		app.setDevicePixelsPerPixel(mainView->devicePixelRatioF());
 
 		const double now = StelApp::getTotalRunTime();
 		double dt = now - previousPaintTime;
@@ -421,7 +418,6 @@ class StelRootItem : public QGraphicsObject
 #endif
 
 		//update and draw
-		StelApp& app = StelApp::getInstance();
 		app.update(dt); // may also issue GL calls
 		app.draw();
 		painter->endNativePainting();

src/core/StelApp.cpp

@@ -444,9 +444,8 @@ void StelApp::init(QSettings* conf)
 	gl = QOpenGLContext::currentContext()->functions();
 	confSettings = conf;
 
-	devicePixelsPerPixel = QOpenGLContext::currentContext()->screen()->devicePixelRatio();
-	if (devicePixelsPerPixel>1)
-		qDebug() << "Detected a high resolution device! Device pixel ratio:" << devicePixelsPerPixel;
+	devicePixelsPerPixel = StelMainView::getInstance().devicePixelRatioF();
+	qDebug() << "Initial high-DPI scaling factor:" << devicePixelsPerPixel;
 
 	setScreenFontSize(confSettings->value("gui/screen_font_size", getDefaultGuiFontSize()).toInt());
 	setGuiFontSize(confSettings->value("gui/gui_font_size", getDefaultGuiFontSize()).toInt());
@@ -1387,6 +1386,7 @@ void StelApp::setDevicePixelsPerPixel(qreal dppp)
 	// Check that the device-independent pixel size didn't change
 	if (!viewportEffect && !fuzzyEquals(devicePixelsPerPixel, dppp))
 	{
+		qDebug() << "Changing high-DPI scaling factor from" << devicePixelsPerPixel << "to" << dppp;
 		devicePixelsPerPixel = dppp;
 		StelProjector::StelProjectorParams params = core->getCurrentStelProjectorParams();
 		params.devicePixelsPerPixel = devicePixelsPerPixel;

src/gui/StelGuiItems.cpp

@@ -386,7 +386,7 @@ void StelButton::paint(QPainter* painter, const QStyleOptionGraphicsItem*, QWidg
 	 *    rectangles.
 	 *    Our keeping QPixmap alive instead of deleting it on return from this function prevents this.
 	 */
-	const double ratio = QOpenGLContext::currentContext()->screen()->devicePixelRatio();
+	const double ratio = StelApp::getInstance().getDevicePixelsPerPixel();
 	if(scaledCurrentPixmap.isNull() || ratio != scaledCurrentPixmap.devicePixelRatioF())
 	{
 		const auto size = boundingRect().size() * ratio;