QOffscreenSurface: Suppress setting of a default geometry on the window

When investigating the bug report, it was discovered that the
offscreen window is assigned a default geometry by
QPlatformWindow::initialGeometry(), causing subsequent resize events
and flushing of event queues. Suppress that by making it a popup which
is not subject to window title bar restrictions on Windows and setting
the respective flags.

Task-number: QTBUG-74176
Change-Id: I7f9c1a3bfd57072c8188a98124bde87491dd25eb
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

1 files changed, 8 insertions, 0 deletions

diff --git a/src/gui/kernel/qoffscreensurface.cpp b/src/gui/kernel/qoffscreensurface.cpp
index ae027af627..0cc11ca3bb 100644
--- a/src/gui/kernel/qoffscreensurface.cpp
+++ b/src/gui/kernel/qoffscreensurface.cpp
@@ -46,6 +46,8 @@
 #include "qwindow.h"
 #include "qplatformwindow.h"
 
+#include <private/qwindow_p.h>
+
 QT_BEGIN_NAMESPACE
 
 /*!
@@ -199,12 +201,18 @@ void QOffscreenSurface::create()
             if (QThread::currentThread() != qGuiApp->thread())
                 qWarning("Attempting to create QWindow-based QOffscreenSurface outside the gui thread. Expect failures.");
             d->offscreenWindow = new QWindow(d->screen);
+            // Make the window frameless to prevent Windows from enlarging it, should it
+            // violate the minimum title bar width on the platform.
+            d->offscreenWindow->setFlags(d->offscreenWindow->flags()
+                                         | Qt::CustomizeWindowHint | Qt::FramelessWindowHint);
             d->offscreenWindow->setObjectName(QLatin1String("QOffscreenSurface"));
             // Remove this window from the global list since we do not want it to be destroyed when closing the app.
             // The QOffscreenSurface has to be usable even after exiting the event loop.
             QGuiApplicationPrivate::window_list.removeOne(d->offscreenWindow);
             d->offscreenWindow->setSurfaceType(QWindow::OpenGLSurface);
             d->offscreenWindow->setFormat(d->requestedFormat);
+            // Prevent QPlatformWindow::initialGeometry() and platforms from setting a default geometry.
+            qt_window_private(d->offscreenWindow)->setAutomaticPositionAndResizeEnabled(false);
             d->offscreenWindow->setGeometry(0, 0, d->size.width(), d->size.height());
             d->offscreenWindow->create();
         }