For Ubuntu 18.04 default Gnome desktop, this quick tutorial is going to show you how to integrates maximized application window with the top panel, by removing the title bar, and moving the title and buttons to the top panel.
All the work can be done via Gnome Shell extension called No Title Bar.
1. Open Ubuntu Software, search for and install No Title Bar extension.
2. The extension takes effect once you installed it. To tweak the appearance, click Extension Settings in Ubuntu Software.
The extension does not work for Gnome 3.30 at the moment, Ubuntu 18.10 users can build it from the source code.