diff -r 169b30f282bd -r a0ddbc9a4e77 src/main.cpp
--- a/src/main.cpp	Tue Jun 28 14:24:00 2022 +0300
+++ b/src/main.cpp	Tue Jun 28 14:53:22 2022 +0300
@@ -28,7 +28,7 @@
 	Q_OBJECT
 public:
 	const ModelId modelId;
-	ModelSubWindow(ModelId modelId, QWidget* widget) :
+	explicit ModelSubWindow(ModelId modelId, QWidget* widget = nullptr) :
 		QMdiSubWindow{widget},
 		modelId{modelId}
 	{
@@ -400,6 +400,19 @@
 	dialog.exec();
 }
 
+template<class SubWindow, class... Args>
+SubWindow* createSubWindow(QMdiArea* mdiArea, Args&&... args)
+{
+	// Qt seems to have a bug where the first created sub window does not render
+	// properly until it is minimized and maximized again. This only happens
+	// if we give the mdi area as a parent argument. As a work-around, we create
+	// the sub window with parent=nullptr, and add it manually.
+	// c.f. https://bugreports.qt.io/browse/QTBUG-69495
+	SubWindow* subWindow = new SubWindow{args..., nullptr};
+	mdiArea->addSubWindow(subWindow);
+	return subWindow;
+}
+
 int main(int argc, char *argv[])
 {
 	doQtRegistrations();
@@ -568,8 +581,9 @@
 			});
 			QObject::connect(data->canvas.get(), &PartRenderer::message, &messageLog, &MessageLog::addMessage);
 			const QFileInfo fileInfo{*documents.modelPath(modelId)};
-			ModelSubWindow* subWindow = new ModelSubWindow{modelId, ui.mdiArea};
+			auto* const subWindow = createSubWindow<ModelSubWindow>(ui.mdiArea, modelId);
 			subWindow->setMinimumSize({96, 96});
+			subWindow->resize({320, 200});
 			subWindow->setWidget(data->canvas.get());
 			subWindow->setWindowTitle(tabName(fileInfo));
 			subWindow->show();
@@ -591,7 +605,7 @@
 	});
 	QObject::connect(ui.actionSettingsEditor, &QAction::triggered, [&]{
 		if (ui.mdiArea->findChildren<SettingsEditor*>().isEmpty()) {
-			SettingsEditor* settingsEditor = new SettingsEditor{defaultKeyboardShortcuts, ui.mdiArea};
+			auto* const settingsEditor = createSubWindow<SettingsEditor>(ui.mdiArea, defaultKeyboardShortcuts);
 			QObject::connect(&settingsChanged, &Signal::triggered, settingsEditor, &SettingsEditor::loadSettings);
 			QObject::connect(settingsEditor, &SettingsEditor::settingsChanged, restoreSettings);
 			settingsEditor->setAttribute(Qt::WA_DeleteOnClose);