Vivaldi does not properly handle the absence of free disk space
My disk partition with the home dir where Vivaldi stores the user profile had accidentally filled up. While I moved the garbage out and tried to open a tab, Vivaldi crashed and lost all opened tabs.
There are some common techniques to prevent configuration corruption on unsuccessful write. For example, write the updated configuration in temporary file and then move it in place of original, or make an automatic backup of overwritten file so it can be restored after a fault. Does Vivaldi use something like this for