Open tab should be highlighted with much visible silver tone colour, keep closing wrong tab, very annoying.
Also "x" to close tab should have been on the right, why ? When the tabs open towards right, why on earth you'd put the close "x" on the left side ?
I love opera and the dark theme but these two things needs must be corrected.
I tried it on Ubuntu (18.04.2 LTS) and it works as desired. So it may well be a macos specific issue.
In fact, I believe that, in macos, the app code may need to specifically direct change-window-focus clicks to the app if it wishes the app to act on the click beyond the change of window focus.
Obviously possible though, given Chrome, Firefox and Safari all do it.
I am running macos 10.14.4 (Mojave).