Fix issue flameshot-org#564, that in wayland fractional scaling situtation, screenshot preview size is wrong. (flameshot-org#3869)

* fix screenshot's size not match with the screen when applying fractional scaling in KDE plasma desktop in wayland.

* Fix: Really fix wayland dpr is not correct and that causes our screenshot preview is wrongly scaled issue. This fix supports that we will use dpr in xcb platform because it is correct in that case.

* Chore: Use more concise variable names. Add comments to explain context.

* Chore: Fix a typo(physcal -> physical). Change code style to meet the .clang-format requirement.

Author: SFBB

src/utils/screengrabber.cpp

@@ -80,13 +80,36 @@ void ScreenGrabber::freeDesktopPortal(bool& ok, QPixmap& res)
       this);
 
     QEventLoop loop;
-    const auto gotSignal = [&res, &loop](uint status, const QVariantMap& map) {
+    const auto gotSignal = [&res, &loop, this](uint status,
+                                               const QVariantMap& map) {
         if (status == 0) {
             // Parse this as URI to handle unicode properly
             QUrl uri = map.value("uri").toString();
             QString uriString = uri.toLocalFile();
             res = QPixmap(uriString);
-            res.setDevicePixelRatio(qApp->devicePixelRatio());
+
+            // we calculate an approximated physical desktop geometry based on
+            // dpr(provided by qt), we calculate the logical desktop geometry
+            // later, this is the accurate size, more info:
+            // https://bugreports.qt.io/browse/QTBUG-135612
+            QRect approxPhysGeo = desktopGeometry();
+            QRect logicalGeo = logicalDesktopGeometry();
+            if (res.size() ==
+                approxPhysGeo.size()) // which means the res is physical size
+                                      // and the dpr is correct.
+            {
+                res.setDevicePixelRatio(qApp->devicePixelRatio());
+            } else if (res.size() ==
+                       logicalGeo.size()) // which means the res is logical size
+                                          // and we need to do nothing.
+            {
+                // No action needed
+            } else // which means the res is physical size and the dpr is not
+                   // correct.
+            {
+                res.setDevicePixelRatio(res.height() * 1.0f /
+                                        logicalGeo.height());
+            }
             QFile imgFile(uriString);
             imgFile.remove();
         }
@@ -248,3 +271,14 @@ QRect ScreenGrabber::desktopGeometry()
     }
     return geometry;
 }
+
+QRect ScreenGrabber::logicalDesktopGeometry()
+{
+    QRect geometry;
+    for (QScreen* const screen : QGuiApplication::screens()) {
+        QRect scrRect = screen->geometry();
+        scrRect.moveTo(scrRect.x(), scrRect.y());
+        geometry = geometry.united(scrRect);
+    }
+    return geometry;
+}

src/utils/screengrabber.h

@@ -18,6 +18,7 @@ class ScreenGrabber : public QObject
     void freeDesktopPortal(bool& ok, QPixmap& res);
     void generalGrimScreenshot(bool& ok, QPixmap& res);
     QRect desktopGeometry();
+    QRect logicalDesktopGeometry();
 
 private:
     DesktopInfo m_info;