Fix multi monitor offset bug in Qt6 multi-monitor setups for X11 (flameshot-org#4127)

* Fix dual monitor offset bug in Qt6 multi-monitor setups

The issue was caused by using QApplication::primaryScreen() to capture
the entire desktop, but in Qt6 the primary screen is not necessarily
the leftmost screen. This caused coordinate system misalignment in
multi-monitor setups where the primary screen is positioned to the right.

Additionally, the previous approach failed in edge cases where screens
have different heights and the desktop bounding box includes virtual
space that doesn't correspond to any physical screen.

Changes:
- Replace single-screen desktop capture with composite approach
- Capture each screen individually and composite them together
- Position capture widget to cover entire desktop geometry
- Remove devicePixelRatio division that caused coordinate issues
- Handle virtual desktop space by filling with black background

This solution works with any multi-monitor configuration including:
- Primary screen not being leftmost
- Screens with different resolutions and alignments
- Bottom-aligned or top-aligned screen arrangements
- Complex layouts with virtual coordinate space

Fixes flameshot-org#4111

* Fix clang-format issue

---------

Co-authored-by: ionsquare <[email protected]>

Author: ionsquare

src/utils/screengrabber.cpp

@@ -205,11 +205,30 @@ QPixmap ScreenGrabber::grabEntireDesktop(bool& ok)
 #endif
 #if defined(Q_OS_LINUX) || defined(Q_OS_UNIX) || defined(Q_OS_WIN)
     QRect geometry = desktopGeometry();
-    QPixmap p(QApplication::primaryScreen()->grabWindow(
-      wid, geometry.x(), geometry.y(), geometry.width(), geometry.height()));
-    QScreen* screen = qApp->screenAt(QCursor::pos());
-    p.setDevicePixelRatio(screen->devicePixelRatio());
-    return p;
+
+    // Qt6 fix: Create a composite image from all screens to handle
+    // multi-monitor setups where screens have different positions/heights.
+    // This fixes the dual monitor offset bug and handles edge cases where
+    // the desktop bounding box includes virtual space.
+    QPixmap desktop(geometry.size());
+    desktop.fill(Qt::black); // Fill with black background
+
+    QPainter painter(&desktop);
+    for (QScreen* screen : QGuiApplication::screens()) {
+        QRect screenGeom = screen->geometry();
+        QPixmap screenCapture = screen->grabWindow(
+          wid, 0, 0, screenGeom.width(), screenGeom.height());
+
+        // Calculate position relative to desktop top-left
+        QPoint relativePos = screenGeom.topLeft() - geometry.topLeft();
+        painter.drawPixmap(relativePos, screenCapture);
+    }
+    painter.end();
+
+    // Set device pixel ratio based on the primary screen
+    desktop.setDevicePixelRatio(
+      QApplication::primaryScreen()->devicePixelRatio());
+    return desktop;
 #endif
 }
 
@@ -259,8 +278,9 @@ QRect ScreenGrabber::desktopGeometry()
 
     for (QScreen* const screen : QGuiApplication::screens()) {
         QRect scrRect = screen->geometry();
-        scrRect.moveTo(scrRect.x() / screen->devicePixelRatio(),
-                       scrRect.y() / screen->devicePixelRatio());
+        // Qt6 fix: Don't divide by devicePixelRatio for multi-monitor setups
+        // This was causing coordinate offset issues in dual monitor
+        // configurations
         geometry = geometry.united(scrRect);
     }
     return geometry;

src/widgets/capture/capturewidget.cpp

@@ -155,7 +155,11 @@ CaptureWidget::CaptureWidget(const CaptureRequest& req,
 #if !defined(FLAMESHOT_DEBUG_CAPTURE)
         setWindowFlags(Qt::BypassWindowManagerHint | Qt::WindowStaysOnTopHint |
                        Qt::FramelessWindowHint | Qt::Tool);
-        resize(pixmap().size());
+        // Fix for Qt6 dual monitor offset: position widget to cover entire
+        // desktop
+        QRect desktopGeom = ScreenGrabber().desktopGeometry();
+        move(desktopGeom.topLeft());
+        resize(desktopGeom.size());
 #endif
 #endif
     }