Ensure that the shell is always visible

The old protocol may not send the default_colors_set message.
Use a timer to make sure that the shell is always visible,
even if no messages are sent by nvim.

src/gui/shell.cpp

@@ -368,6 +368,15 @@ 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 deprecated cell-based grid protocol.
+	QTimer::singleShot(300, [&]() {
+		if (!m_shown) {
+			setVisible(true);
+			m_shown = true;
+		}
+	});
 }
 
 void Shell::neovimError(NeovimConnector::NeovimError err)