Base line of tabs should extend full width of window, and there should be a gap between tabs - to be consistent with all other tabs (e.g. terminal, gedit)
As can be seen in the screenshot the 'base line' of the tabs does not stretch fully to the left (or indeed fully to the right):

And, the following shows that the nautlilus tabs have no gap (the hovered tab is right next to the active tab):

I you installed from the sources, go the Yaru folder and copy the output of the following terminal command instead
`$ git describe`
18.10.4-9-gfe6a55f4
Thanks @CDrummond
This was actually mentioned https://github.com/ubuntu/yaru/pull/745 but I forgot about it - sorry!
This is actually easier to fix than I though. Sorry in advance for the long post, but it needs to be told :D
Initially, Nautilus had a lot of custom code for notebook and the issue above was even more visible because it was in between tabs, so I reduced the padding to zero to "mitigate it". In the latest code review, we even reduced the Nautilus custom code but the issue was apparently still there. Only now I tried to remove completely the custom code and that fixes it XD
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This is actually easier to fix than I though. Sorry in advance for the long post, but it needs to be told :D
Initially, Nautilus had a lot of custom code for notebook and the issue above was even more visible because it was in between tabs, so I reduced the padding to zero to "mitigate it". In the latest code review, we even reduced the Nautilus custom code but the issue was apparently still there. Only now I tried to remove completely the custom code and that fixes it XD