mainwindow: set a sensible default size

When `~/.config/nvim-qt/window-geometry.conf` does not exist then
restoreGeometry() will initialize a window that is tiny and must be
resized to see anything.

Use a sensible default size on the initial load and when we have
disabled restoring window geometry.

Avoid restoring state when it is invalid.

src/gui/mainwindow.cpp

@@ -1,8 +1,10 @@
 #include "mainwindow.h"
 
+#include <QApplication>
 #include <QCloseEvent>
 #include <QEventLoop>
 #include <QLayout>
+#include <QScreen>
 #include <QSettings>
 #include <QStyleFactory>
 #include <QToolBar>
@@ -14,6 +16,19 @@ static QString DefaultWindowTitle() noexcept
 	return "Neovim";
 }
 
+static void SetDefaultWindowSize(QWidget *widget)
+{
+	QPoint widget_position = widget->mapToGlobal(QPoint{ widget->width() / 2, 0 });
+	QScreen *screen = qApp->screenAt(widget_position);
+	QRect geometry{ screen->availableGeometry() };
+	if (screen->orientation() == Qt::LandscapeOrientation) {
+		widget->resize(geometry.width() / 2, geometry.height());
+	}
+	else {
+		widget->resize(geometry.width(), geometry.height() / 2);
+	}
+}
+
 MainWindow::MainWindow(NeovimConnector* c, QWidget* parent) noexcept
 	: QMainWindow{ parent }
 	, m_tabline{ *c, this }
@@ -331,10 +346,20 @@ void MainWindow::restoreWindowGeometry()
 
 	QSettings settings("nvim-qt", "window-geometry");
 	if (!settings.value("restore_window_geometry", true).toBool()) {
+		SetDefaultWindowSize(this);
 		return;
 	}
-	restoreGeometry(settings.value("window_geometry").toByteArray());
-	restoreState(settings.value("window_state").toByteArray());
+	QVariant geometry{ settings.value("window_geometry") };
+	if (geometry.isValid()) {
+		restoreGeometry(geometry.toByteArray());
+	}
+	else {
+		SetDefaultWindowSize(this);
+	}
+	QVariant state{ settings.value("window_state") };
+	if (state.isValid()) {
+		restoreState(state.toByteArray());
+	}
 }
 
 void MainWindow::setGuiAdaptiveColorEnabled(bool isEnabled)