gui: avoid a flashing window blink during startup

The very first paint events cause a bright flash of unstyled shell
content to appear before the configured colors are applied.

Defer displaying the shell widget until the default colors have been
provided by nvim.

src/gui/shell.cpp

@@ -326,6 +326,9 @@ void Shell::init()
 		return;
 	}
 
+	if (!m_shown) {  // Defer displaying the shell until colors have been set.
+		setVisible(false);
+	}
 	connect(m_nvim->api0(), &NeovimApi0::neovimNotification,
 			this, &Shell::handleNeovimNotification);
 	connect(m_nvim->api0(), &NeovimApi0::on_ui_try_resize,
@@ -1127,8 +1130,11 @@ void Shell::handleDefaultColorsSet(const QVariantList& opargs)
 	setBackground(backgroundColor);
 	setSpecial(specialColor);
 
-	// Cells drawn with the default colors require a re-paint
-	update();
+	// Display the shell now that the default colors have been set.
+	if (!m_shown) {
+		setVisible(true);
+		m_shown = true;
+	}
 	emit colorsChanged();
 }
 

src/gui/shell.h

@@ -194,6 +194,7 @@ private slots:
 private:
 	bool m_init_called{ false };
 	bool m_attached{ false };
+	bool m_shown { false };
 	NeovimConnector* m_nvim{ nullptr };
 
 	QList<QUrl> m_deferredOpen;