Vivaldi pops up over other apps.
I've got two monitors and Vivaldi windows open on both. If I open something up on top of the Vivaldi window on the second monitor and then move my mouse onto the Vivaldi window on the first monitor, the Vivaldi window on the second monitor appears, covering the thing I just opened up.
So, for example, I have speed dial open on monitor one and Facebook on monitor two. I open up Thunderbird on monitor two. As I'm moving my mouse over the speed dial window, the Facebook window appears, making it impossible to ever interact with Thunderbird without minimising one or both Vivaldi windows.
This is with Vivaldi 1.13.1008.32 (Stable channel) (64-bit) on Linux Mint 17.3 'Rosa' MATE.