gnome-shell-extension-autohidetopbar - 1:120-1 main

This gnome-shell extension automatically hides the topbar, except in overview.
There is also an option to show the topbar when the mouse pointer approaches
the edge of the screen.
.
Please note that you have to manually enable the extension, for example using
gnome-extensions-app, which is part of the gnome-shell-extension-prefs package.