Ubuntu – hide the title bar of MPlayer in gnome

I solved this by using Maximus.

For newer versions of gnome (> 3.8) you may need to edit metadata.json to match this pull request and manually install the extension. Instructions lifted from maximus homepage with a few edits:

hg clone ssh://hg@bitbucket.org/mathematicalcoffee/maximus-gnome-shell-extension
hg up gnome3.4 # or `stable` if on GNOME 3.2
cd maximus-gnome-shell-extension
# NOW OPEN AND EDIT THE OUTDATED metadata.json FILE. See link above.
make # <-- VERY IMPORTANT
cp -r maximus@mathematical.coffee.gmail.com ~/.local/share/gnome-shell/extensions
# enable maximus if you haven't already:
gnome-shell-extension-tool -e maximus@mathematical.coffee.gmail.com
Restart gnome-shell.

To get to the core of why the metacity theme tweak no longer works we have to look at the gnome blog itself where gnome developer where Florian Müllner explains that when drawing the title bars gnome no longer uses metacity at all and they are always drawn by Gtk+

This means that a css answer is the only one that will work and I believe you are most of the way there. I read on this commit page that the css you want is somewhat like this:

.maximized .header-bar.default-decoration {
  padding: 0;
  font-size: 0.7em;
}

.maximized .header-bar.default-decoration .button.titlebutton {
  padding: 0;
  border-width: 0;
}

The reason the css can't hide or remove the element like it can with html (or svg) is that the css applies to gtk properties and the visibility of an object isn't a property in the typical Gtk way. You actually have to call widget.hide() from code. Similar to how Gimp themes have to hide widgets by making them the same colour as the background as a hack.