When a KMessageWidget appears, the sizeHint width of the MainView grows up a lot.
So it produces bigger tabs with an incorrect position of the addTabButton. (eg: Try to log in to Gmail)
I fixed this issue by setting a fixed max value for tab width. (MainView default width without a KMessageWidget)
I haven't found a cleaner way to do that.
I also removed a check which broke the addTabButton position in MainView.