Bookmarks sidebar ...
a) remove the triangle - it serves no purpose - it just takes up a lot of space (need to change so clicking on the description or folder opens with one click)
b) when r-click a bookmark add option to "Change to current" - this is very useful if just used the bookmark but the URL no longer leads to a page and perhaps have to chop back the URL to the previous forward slash for example (note a previous request from me to add an Up Arrow which chops back when hit ... then just need to Change to Current ...)
The first feature is already on the list in the Feature Requests thread. The latter, I believe, is not there yet, so you might consider adding it there instead, as it's a very good idea. While I'm not sure whether the Vivaldi devs have enough time to read each and every thread on the forums, I believe they pay close attention to the Feature Requests threads, as they keep delivering many of those features with new Snapshot updates... 😉