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 nvim sends us colors.

Instead of seeing the shell with its constructor-provided colors
we will instead see Qt's default window background color, which
follows the user's desktop theme and dark mode settings.

Once nvim sends a color event then the shell widget is made visible
after the colors are applied.

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;