screengrabber: pass non-empty parent_window to xdg-desktop-portal (flameshot-org#4664)

* screengrabber: pass non-empty parent_window to xdg-desktop-portal

xdg-desktop-portal-gnome on GNOME 46 rejects requests where the
parent_window argument is an empty string with the error
"Failed to associate portal window with parent window ''",
which made daemon-mode (tray icon, DBus captureScreen) capture
fail for all GNOME Wayland users.

Allocate an offscreen QWidget (Qt::WA_DontShowOnScreen) so a
winId is generated, and pass "x11:<hex>" on X11 sessions or
"wayland:" on Wayland. Both are accepted by xdg-desktop-portal
as valid no-real-parent handles, while remaining non-empty as
the portal spec requires.

Tested on: Ubuntu 24.04, GNOME 46 Wayland, NVIDIA proprietary
590.48.01, Qt 6.4.2, xdg-desktop-portal-gnome 46.2.

Fixes daemon-mode capture on GNOME 46 Wayland.

* clang format

---------

Co-authored-by: Jeremy Borgman <borgman.jeremy@pm.me>

Author: artefaktor93

src/utils/screengrabber.cpp

@@ -111,9 +111,28 @@ void ScreenGrabber::freeDesktopPortal(bool& ok, QPixmap& res)
     });
     timeout.start();
 
+    // Build a non-empty parent_window handle. xdg-desktop-portal-gnome
+    // (>= 46) rejects an empty string with "Failed to associate portal
+    // window with parent window ''" and the Screenshot request fails.
+    // On X11 we pass a real x11:<hex> handle from an offscreen QWidget.
+    // On Wayland we fall back to an empty string inside a wayland: prefix
+    // (xdg-desktop-portal-gnome treats unknown handles as no-parent and
+    // proceeds, instead of rejecting outright).
+    QString parentWindow;
+    QWidget parentDummy;
+    parentDummy.setAttribute(Qt::WA_DontShowOnScreen, true);
+    parentDummy.resize(1, 1);
+    parentDummy.show();
+    if (QGuiApplication::platformName() == QLatin1String("wayland")) {
+        parentWindow = QStringLiteral("wayland:");
+    } else {
+        parentWindow =
+          QStringLiteral("x11:0x%1").arg(parentDummy.winId(), 0, 16);
+    }
+
     screenshotInterface.call(
       QStringLiteral("Screenshot"),
-      "",
+      parentWindow,
       QMap<QString, QVariant>({ { "handle_token", QVariant(token) },
                                 { "interactive", QVariant(false) } }));