unfocussed tabs are transparent

You mean you don't want kittens peeping through bits of your window? You monster! :-D

Okay, so I'm fairly certain you got bitten by zesty having 3.22 gtk. Gnome, in their infinite (and absolutely always correct) wisdom decided to change CSS handling in gtk 3.20, breaking applications that were using the class to assign styling. From 3.20 it only works with node names. We actually already have a fix in http://bazaar.launchpad.net/~gnome-terminator/terminator/gtk3/revision/1714 but you are ~10 revisions behind. You could enable the nightly gtk3 PPA found at https://code.launchpad.net/~gnome-terminator/+archive/ubuntu/nightly-gtk3 which will get you all the latest fixes.

Be aware that rev 1714 is only a partial fix, as the theme specific fixes and styling are very likely also broken. It looks like you are using the Ambiance theme by the styling of the tabs in your image, but with Ambiance you *should* see tabs that look like the ones in gnome-terminal if you have the "Extra Styling" checked in Preferences.

I cannot see this issue (I'm on gtk 3.18) so I'm rather reliant on someone being willing to either fix these themselves, or be prepared to do a frequent and quick feedback loop, being my eyes while I make changes.

@stephen-j-boddy you should be able to repeat this out of the box with an Ubuntu 17.04 vm.

Or, you know, you could test a newer version of Terminator which I already stated has a fix for all the transparency issues... like I mentioned in comment #3. That's a lot easier than me upgrading my system to a non-LTS release so I can confirm something you can do in 5 minutes. If you don't want switch to nightly, you can run a quick test without affecting the current install.

Make sure no other Terminator instances are running. Use another terminal to run the following commands.

You'll need bzr if you don't have it:
sudo apt-get install bzr

Then create a local copy:
bzr branch lp:terminator <TARGET>

which will create the folder TARGET containing the latest dev code. In case not obvious, replace TARGET with a suitable name, also removing the <>. Then cd into it, and run the branch of terminator:
cd <TARGET>
./terminator

Check tabs.

Report back.

stephen-j-boddy yup, looks like it's fixed in latest dev.

Marking as Invalid as the fix was already released, and no action to be taken.

Status changed to 'Confirmed' because the bug affects multiple users.

I'm also running 17.04 and am having the same issue.
I've also done what stephen-j-boddy suggested to get the dev version and problem solved!

Summary:
~$ cd ~/bin
~/bin$ bzr branch lp:terminator terminator-dev
~/bin$ ln -s ~/bin/terminator-dev/terminator
# close any open terminator instances now before trying the new version

Since ~/bin is in the front of $PATH, it will start the dev version of terminator before the default one installed with apt. It seems that using the icons in unity/gnome also fire up the dev version now.

Until a fix is released in 17.04, this is my workaround.

Still have this problem with latest terminator version

Have the same issue, all packages are current (as of today):

$ terminator --version
terminator 1.90

This is terribly annoying :(

OK, not sure why I thought setting the Ubuntu package to Invalid was a good idea. Not my call, as I don't deal with the official Ubuntu packaging myself. (There are nightly builds I produce, but they're not Ubuntu official.)

Same issue with terminator 1.90.

Workaround described in the bug description <a href="https://bugs.launchpad.net/ubuntu/+source/terminator/+bug/1676082">here</a> solved the problem.