This doesn't happen every time for me, but when it happens, we basically stop drawing the window title and shift everything up without making room for the mac window buttons on the top left side. Repainting the window doesn't seem to help things, but clicking on the green button to maximize and clicking it again to toggle back to the original size seems to bring the titlebar back.

Depends on or dupe of Bug 930094?

Hard to say.

The very same thing happens when you customize UI. Steps to reproduce: 1. Choose Customize and Control Nightly > Customize OR Choose Customize from toolbar context menu 2. Choose Exit Customize There a way to bring back the title bar, though. You have to resize window making it bigger. It's either just grow the size of the window or unmaximize and maximize it back.