Note: this is a mac-only issue.

Steps to reproduce:

• Put the game in fullscreen using mac's fullscreen button (top right in maverics, green button top left in yosemite)
• While the game is in fullscreen, press F11 (the old fullscreen button)
• Press F11 again to disable fullscreen
• Press the mac's fullscreen button again to go into mac fullscreen

It does not always happen the first time you try it. Try these steps a few times after each other and you'll get the issue. When going back into mac fullscreen (last step), the game doesn't seem to update its resolution, thus showing a black bar on the top of the game window. This issue has been present since the mac fullscreen support got added in 1.7.6.
My suggestion for fixing this would be either disabling the F11 button while the game is in mac fullscreen, or properly refreshing the resolution when the game goes into mac fullscreen.

See the screenshot below for an example