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.

The shell is made visible either after nvim sends a "default_colors_set"
message and the colors are applied, or when the visibility timer fires,
whichever happens first. In practice, the "default_colors_set" message
is typically received before the timer times out and runs its callback.

src/gui/shell.cpp

@@ -103,6 +103,15 @@ Shell::Shell(NeovimConnector *nvim, QWidget *parent)
 	m_nvim->setRequestHandler(new ShellRequestHandler(this));
 }
 
+void Shell::ensureVisible() noexcept
+{
+	if (!m_shown) {
+		m_shown = true;
+		setVisible(true);
+		setFocus();
+	}
+}
+
 void Shell::handleFontError(const QString& msg)
 {
 	if (m_attached) {
@@ -326,6 +335,10 @@ void Shell::init()
 		return;
 	}
 
+	// Pull Request#1101: Defer displaying the shell until colors have been set
+	if (!m_shown) {
+		setVisible(false);
+	}
 	connect(m_nvim->api0(), &NeovimApi0::neovimNotification,
 			this, &Shell::handleNeovimNotification);
 	connect(m_nvim->api0(), &NeovimApi0::on_ui_try_resize,
@@ -365,6 +378,16 @@ void Shell::init()
 
 	// Set initial value
 	m_nvim->api0()->vim_set_var("GuiWindowFrameless", (windowFlags() & Qt::FramelessWindowHint) ? 1: 0);
+
+	// Make the shell visible even when default_colors_set is not received,
+	// e.g. when using the older cell-based grid protocol.
+	if (!m_shown) {
+		constexpr int visibility_timeout{ 850 };
+		m_visibility_timer.setInterval(visibility_timeout);
+		m_visibility_timer.setSingleShot(true);
+		connect(&m_visibility_timer, &QTimer::timeout, this, &Shell::ensureVisible);
+		m_visibility_timer.start();
+	}
 }
 
 void Shell::neovimError(NeovimConnector::NeovimError err)
@@ -1127,6 +1150,8 @@ void Shell::handleDefaultColorsSet(const QVariantList& opargs)
 	setBackground(backgroundColor);
 	setSpecial(specialColor);
 
+	// Display the shell now that the default colors have been set.
+	ensureVisible();
 	// Cells drawn with the default colors require a re-paint
 	update();
 	emit colorsChanged();

src/gui/shell.h

@@ -190,10 +190,12 @@ protected slots:
 
 private slots:
 	void setAttached(bool attached);
+	void ensureVisible() noexcept;
 
 private:
 	bool m_init_called{ false };
 	bool m_attached{ false };
+	bool m_shown{ false };
 	NeovimConnector* m_nvim{ nullptr };
 
 	QList<QUrl> m_deferredOpen;
@@ -231,6 +233,8 @@ private slots:
 	Qt::MouseButton m_mouseclick_pending;
 	// Accumulates remainder of steppy scroll
 	QPoint m_scrollDeltaRemainder;
+	// Ensures that the Shell widget is made visible
+	QTimer m_visibility_timer;
 
 	// Properties
 	bool m_neovimBusy{ false };