Fix X11 multi-monitor capture on HiDPI (screen cut/shift, inflated canvas)

x11LegacyScreenshot() multiplied the united logical geometry by a single
devicePixelRatio and drew each screen at its logical top-left. When the
ratio is not 1 and the monitors have different sizes, the composite is wrong.

On the xcb backend the virtual-desktop layout uses native (device) pixel
offsets, while each screen reports a logical size (its device size divided by
its ratio). Multiplying the whole union by one ratio double-counts the
offsets: a 7680x3996 desktop is captured as 10560x5373, and the region the
GUI overlay selects is cut or shifted onto the wrong monitor.

grabWindow(0) already returns the device-pixel pixmap of each screen, and
geometry().topLeft() is the native offset on xcb. So draw each pixmap at its
native offset using its own physical size, and never scale. The canvas is the
union of those device-pixel rectangles.

Tested on X11/awesome, nvidia driver, Xft.dpi 168 (ratio 1.75), three
monitors of two sizes (3264x1836 and two 3840x2160 in an L shape). Capture is
now pixel-aligned to the framebuffer (per-monitor RMSE < 0.02), against
10560x5373 before.

Author: manuel-delverme

src/utils/screengrabber.cpp

@@ -870,26 +870,29 @@ QPixmap ScreenGrabber::x11LegacyScreenshot()
         return p;
     }
 
-    // Composite all screens using logical geometry.
-    // On i3 (tested) DPR is uniform so we don't need the per-screen
-    // physical pixel math that the Windows backend does. Not sure if this is
-    // true for other DE's like xmonad.
-    QRect totalGeom;
+    // Composite in device pixels. On xcb the virtual-desktop layout uses native
+    // offsets while each screen reports a logical size, so scaling the united
+    // geometry by one ratio inflates and misplaces it. grabWindow(0) returns the
+    // device-pixel pixmap and geometry().topLeft() is the native offset, so draw
+    // each pixmap at its native offset using its own size.
+    QRect totalRect;
+    QList<QPair<QRect, QPixmap>> grabs;
+    grabs.reserve(screens.size());
     for (QScreen* s : screens) {
-        totalGeom = totalGeom.united(s->geometry());
+        QPixmap p = s->grabWindow(0);
+        p.setDevicePixelRatio(1.0);
+        QRect r(s->geometry().topLeft(), p.size());
+        grabs.append({ r, p });
+        totalRect = totalRect.united(r);
     }
 
-    qreal dpr = screens.first()->devicePixelRatio();
-    QPixmap desktop(qRound(totalGeom.width() * dpr),
-                    qRound(totalGeom.height() * dpr));
-    desktop.setDevicePixelRatio(dpr);
+    QPixmap desktop(totalRect.size());
+    desktop.setDevicePixelRatio(1.0);
     desktop.fill(Qt::black);
 
     QPainter painter(&desktop);
-    for (QScreen* s : screens) {
-        QPixmap p = s->grabWindow(0);
-        QPoint offset = s->geometry().topLeft() - totalGeom.topLeft();
-        painter.drawPixmap(offset, p);
+    for (const auto& g : grabs) {
+        painter.drawPixmap(g.first.topLeft() - totalRect.topLeft(), g.second);
     }
     painter.end();