desktop: update comment to reflect reason for QQuickItem destruction

Explain the ominous behavior that leads to the StatsView being
destroyed under our feet by Qt6.

Also, don't generate the view in the destructor. No point in
doing that, when we have to regenerate it anyway for each
access.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

core/subsurfacestartup.c

@@ -91,6 +91,7 @@ static void print_help()
 	printf("\n --dc-product=product  Set the dive computer to download from");
 	printf("\n --device=device       Set the device to download from");
 #endif
+	printf("\n --no-opengl-sharing   Don't share OpenGL contexts between QQuickItems (for testing)");
 	printf("\n --cloud-timeout=<nr>  Set timeout for cloud connection (0 < timeout < 60)\n\n");
 }
 
@@ -149,6 +150,10 @@ void parse_argument(const char *arg)
 				++force_root;
 				return;
 			}
+			if (strcmp(arg, "--no-opengl-sharing") == 0) {
+				// This was already checked in main()
+				return;
+			}
 #if SUBSURFACE_DOWNLOADER
 			if (strncmp(arg, "--dc-vendor=", sizeof("--dc-vendor=") - 1) == 0) {
 				prefs.dive_computer.vendor = strdup(arg + sizeof("--dc-vendor=") - 1);

desktop-widgets/statswidget.cpp

@@ -91,19 +91,24 @@ StatsWidget::StatsWidget(QWidget *parent) : QWidget(parent)
 
 	ui.stats->setSource(urlStatsView);
 	ui.stats->setResizeMode(QQuickWidget::SizeRootObjectToView);
-	(void)getView();
 }
 
-// hack around the Qt6 bug where the QML object gets destroyed and recreated
+// The Qt-developers do not have their ownership management under control.
+// This is not surprising to anyone who has looked at their source code.
+// In contradiction to the documentation, starting with Qt6, QQuickItems
+// _will_ be deleted when the parent widgets are reparented, even when
+// CppOwnership is set. This can be prevented for OpenGL backends, but not
+// for others. Therefore, we have to _always_ refetch the view from the QML page.
+// For details and precise conditions, see comments and code in
+// QQuickWidgetPrivate::handleWindowChange().
+// Ultimately, we will have to change the way widgets are hidden/shown in
+// the MainWindow.
 StatsView *StatsWidget::getView()
 {
 	StatsView *view = qobject_cast<StatsView *>(ui.stats->rootObject());
 	if (!view)
 		qWarning("Oops. The root of the StatsView is not a StatsView.");
 	if (view) {
-		// try to prevent the JS garbage collection from freeing the object
-		// this appears to fail with Qt6 which is why we still look up the
-		// object from the ui.stats rootObject
 		ui.stats->engine()->setObjectOwnership(view, QQmlEngine::CppOwnership);
 		view->setParent(this);
 		view->setVisible(isVisible()); // Synchronize visibility of widget and QtQuick-view.

subsurface-desktop-main.cpp

@@ -13,6 +13,7 @@
 #include "core/qt-gui.h"
 #include "core/qthelper.h"
 #include "core/subsurfacestartup.h"
+#include "core/subsurface-string.h"
 #include "core/settings/qPref.h"
 #include "core/tag.h"
 #include "desktop-widgets/mainwindow.h"
@@ -38,9 +39,22 @@ int main(int argc, char **argv)
 	if (verbose) /* print the version if the Win32 console_init() code enabled verbose. */
 		print_version();
 
-	int i;
 	bool no_filenames = true;
 	QLoggingCategory::setFilterRules(QStringLiteral("qt.bluetooth* = true"));
+
+	// Allow OpenGL-based QtQuick backends to share OpenGL contexts.
+	// This avoids destruction of QQuickItems if their parent widgets are moved between widgets in Qt6.
+	// Sadly, this does not help when running on non-OpenGL backends.
+	// Allow for turning this off to test destruction of QQuickItems.
+	// The parameter is checked here because QCoreApplication::parameters() only
+	// works after instantiation of the QApplication class, but the flag has
+	// to be set bofore.
+	if (std::any_of(argv + 1, argv + argc,
+	    [](char *arg) { return same_string(arg, "--no-opengl-sharing"); })) {
+		fprintf(stderr, "Disabling sharing of OpenGL contexts\n");
+	} else {
+		QGuiApplication::setAttribute(Qt::AA_ShareOpenGLContexts);
+	}
 	std::unique_ptr<QApplication> app(new QApplication(argc, argv));
 	QStringList files;
 	QStringList importedFiles;
@@ -50,7 +64,7 @@ int main(int argc, char **argv)
 	const char *default_filename = system_default_filename();
 	subsurface_mkdir(default_directory);
 
-	for (i = 1; i < arguments.length(); i++) {
+	for (int i = 1; i < arguments.length(); i++) {
 		QString a = arguments.at(i);
 		if (a.isEmpty())
 			continue;