Settings window appear on wrong monitor
When I go "Tools -> Settings" the settings window opens on another monitor; a monitor which is neither primary nor contains the (only) open Vivaldi instance. If I move the settings window to the primary monitor, where the Vivaldi instance is open, close it then reopen the settings window again, it shows up again at the wrong monitor. Linux Mint 18 Cinnamon. Happens regardless of whether "Attach dialog windows to the parent window" is selected or not in the Cinnamon Windows Behavior setting screen. "Use Native Window" is not selected in Vivaldi.
I have this issue as well. Also on Linux Mint 18.
I don't think this is entirely a Vivaldi issue. A few other programs do this as well. Usually it seems to be when there's a popup window (very often a settings window).
Do you have your primary monitor set to the right side? That's how I have mine. Some programs seem to want to open windows up on the left monitor, even if it is set as a secondary monitor.
When looking into this issue, I found this: http://segfault.linuxmint.com/2015/08/configuring-the-mdm-login-screen-for-multiple-monitors/
It didn't work for me, but maybe it will for you.
I have three monitors, primary with Vivaldi in the middle, settings appear to the left.
Similar symptom then. The primary monitor is not the left-most monitor.
Have you noticed other programs having some windows pop up on the left side?
For me, Password Safe does it. Opera 12 does it, if you have the dialog box open up on start-up. Okular does it when I view a PDF in 'presentation mode'.
The workaround for Vivaldi would be to Check 'Settings –> Appearance --> Window Appearance --> Open Settings in a Tab'