Lightroom CC2015.12/6.12 was released today and should address this issue. Please update your Lightroom to the latest version and let us know if you continue to see the issue. Thank you for your patience.
I continue to have this problem after spending over two hours online with support today. I was told that it would be fixed in an update that is coming out in the next few days. Resetting preferences and unchecking the show tab bar doesn't help.
This issue is caused by when "Show Tab Bar" (Window -> Show Tab Bar) is enabled in Lightroom.
To resolve this, disable "Show Tab Bar" (Window -> Show Tab Bar) and restart Lightroom. You can also use the shortcut CMD+SHIFT+T to toggle between these.
As Terri mentioned, unchecking tab bar or resetting preferences only solves it once, but it happens again and again. Am using imac 2014 OS Sierra and latest LR CC.
This is really annoying as I use LR on a daily basis for work. So resetting it is time consuming as I also need to reset the export dpi to 300 instead of the default 240. If adobe can give me a better temp fix than resetting the preferences each time that would save me time.
Just updated LR and still have this problem. Unchecking tab bar still does not work, even when I restart LR. In fact, after restarting LR, the tab bar is checked again, even after I've unchecked it. Very frustrating. I am paying monthly for a service that is not performing as expected.
I had to go thru the suggested twice as my "show tab bar" initially showed as off (disabled). Problem re appeared after restarting LR after first try. Second try of disable and restart worked. Thanks very much. ((I don't see where to click that this is a correct answer? But it is correct for me.))
So when is the update coming? I tried not showing the tool bar and restarting lightroom. Changing preferences doesn't really work. It is really weird because I used Lightroom yesterday and I come on today to see it all wonky. I haven't changed any settings so it must be lightroom that is having the issue. Ugh so frustrating trying to get these pictures processed.
Latest LR update to 2015.10.1 does not fix this problem. After updating, I've tried the solutions suggested above, unchecking tab bar and resetting preferences. Neither of these fix it for me.
Most be quirky to particular OS? I have Mac Sierra. Unchecking tab bar has worked for me. Even after a restart and an couple of LR reinstalls due to an SSD upgrade it has always stayed unchecked.