I think I just had the same problem and I solved it by switching the theme.
Go to Preferences -> General -> Appearance and switch to the Classic theme.
I can’t understand why this helps, but it worked for me.
Update:
After being bitten by this again I checked again and as @anweibel says in his answer this can be edited in the CSS.
I just installed the E4 CSS editor plugin using the Eclipse Marketplace so that I could change the CSS directly from Eclipse under Preferences -> Apperance. I changed swt-mru-visible from false to true so that the relevant part for the GTK theme looked like this:
.MPartStack {
font-size: 11;
swt-simple: false;
swt-mru-visible: true;
}
A restart was required for the changes to take effect.
More info can also be found here: Eclipse tabs repositionning