invisible "project tab bar"

Prerequisites

[x] Put an X between the brackets on this line if you have done all of the following:
- Reproduced the problem in Safe Mode: http://flight-manual.atom.io/hacking-atom/sections/debugging/#using-safe-mode
- Followed all applicable steps in the debugging guide: http://flight-manual.atom.io/hacking-atom/sections/debugging/
- Checked the FAQs on the message board for common solutions: https://discuss.atom.io/c/faq
- Checked that your issue isn't already filed: https://github.com/issues?utf8=✓&q=is%3Aissue+user%3Aatom
- Checked that there is not already an Atom package that provides the described functionality: https://atom.io/packages

Description

An semi-invisible "project tab bar" appears under 1.18.0 when I start multiple instances of Atom. I can click on this to switch between different instances of Atom but this distorts the rendering of the first line in the editor.

Screenshot:

I've tried safe-mode, and also wiping the saved state information, didn't seem to help.

Steps to Reproduce

cd ~/a; atom .
(in another terminal) cd ~/b; atom .
the new Atom shows this symptom

Expected behavior: No invisible tab bar

Actual behavior: Invisible tab bar appears that seems to be functional but not quite rendered.

Reproduces how often: 100%

Versions

% atom --version ; apm --version
Atom    : 1.18.0
Electron: 1.3.15
Chrome  : 52.0.2743.82
Node    : 6.5.0
apm  1.18.1
npm  3.10.10
node 6.9.5 x64
python 2.7.11
git 2.7.3

OS: Mac Sierra

Additional Information

Any additional information, configuration or data that might be necessary to reproduce the issue.

atom / tabs