Auto hide your Atom tab bar if only one tab is open.

auto hide tabs.coffee

# Put this in your init script (Atom ➞ Open Init Script)

tabs = atom.workspaceView.find('.tab-bar')

tabs.on "DOMNodeInserted", ->
  # find each tab bar
  if (tabs.find('li').length) > 1
    tabs.slideDown(300)
  else
    tabs.slideUp(300)

You can accomplish this with styles too if you so desire.

Add this to your style sheet (Atom ➞ Open Your Stylesheet)

.tab-bar li:only-child {
  display: none !important;
}

The script approach seems to have bit-rotted and the CSS approach doesn't actually collapse the tab bar area -- it simply hides the tab UI element. Any updated versions of this?

F#$k both methods, first one does not work, second one only hides the tabs, not the tab bar, which I find insanely stupid. Shame on you @andymcintosh for such a retarded solution

Found it.
1. Atom > Preferences
2. Click on "Packages"
3. Search for "tabs" package
4. Click the "Settings" button for the "tabs" package.
5. Untick "Always Show Tab Bar"
And the styling solution to really hide tab bar:
.tab-bar { height: 0px; visibility: hidden; }