So, the three dots to the right of the extensions hides all of them and makes all of the re-appear. But when you right-click on each one, you are able to "hide" extensions. In Chrome, these queue up and when you press the button on Chrome that is equivalent to the three dots on Vivaldi, it opens up a menu and you can select those hidden extensions. I do not see this function with the three dots.
Is it located somewhere else?
Otherwise a) how can I get back the extensions I have "hidden" and b) why isn't there (or rather, can you please implement) a similar menu so that less-used extensions can be stowed away while the everyday use extensions can remain as they are?
Open vivaldi://extensions (may be Cmd+Shift+E may work too) and in extension list enable to Show button