@vux777 Yes, my idea was not good !

Will we have to wait for the return of the flag #sidebar-extensions-in-browser-sidebar in the future versions of Opera ...

@vux777 A solution to reduce the width of the sidebar would be to be able to remove the search box (in Settings).

In the latest version of Opera (69.0.3686.95), I can no longer reduce the addon's width all the way to the left (bring it as close to the sidebar as possible). It can only increase in size to the right.
the previous solution was to disable the flag opera://flags/#sidebar-extensions-in-browser-sidebar but there is no longer this flag.
Is there a new solution ?