I see the same as Alice on Ubuntu 18.04 without moving anything into the tab bar, simply having the system titlebar enabled causes the tab bar click zones to be vertically misaligned. Ubuntu 18.10 is unaffected.
Bug 1525850 Comment 4 Edit History
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I see the same as Alice on Ubuntu 18.04 without moving anything into the tab bar, simply having the system titlebar enabled causes the tab bar click zones to be vertically misaligned. ~~Ubuntu 18.10 is unaffected.~~