create a file ~/.config/gtk-3.0/gtk.css ( add the below CSS )
then you will need to reload gnome-shell: ALT + F2 and type r
I was able to reduce the app Titlebar on Gnome 3.20 with the following CSS:

headerbar entry,
headerbar spinbutton,
headerbar button,
headerbar separator {
    margin-top: 0px; /* same as headerbar side padding for nicer proportions */
    margin-bottom: 0px;
}

headerbar {
    min-height: 24px;
    padding-left: 2px; /* same as childrens vertical margins for nicer proportions */
    padding-right: 2px;
    margin: 0px; /* same as headerbar side padding for nicer proportions */
    padding: 0px;
  }

beaucoup moins buggy que l'originale

Une nouvelle ext gnome via VG thx :)

Upgrading Ubuntu GNOME

To Upgrade Or Not To Upgrade?

Before upgrading, it is highly recommended to back up your data. See this page for tutorials on how to backup your data. Or just download Clonezilla and create an image of your hard drive or system partition.

The included GUI update manager will offer you the latest release of Ubuntu GNOME. Which can also be accessed via Terminal:

sudo update-manager -d

Alternatively, launch Terminal by hitting Ctrl+Alt+T and run the following command:

sudo do-release-upgrade

Note. If you were using the gnome3-team/gnome3-staging PPA, you should run

sudo ppa-purge ppa:gnome3-team/gnome3-staging

before upgrading.

If you are having problems with upgrading - No new release found - please see this.

More information about Upgrading can be found here.