That's really sad!! Totally agree that native Opera's sidebar is unusable.
Really no workaround? Half a year ago there was a similar situation and this here did help: https://forums.opera.com/post/195641 .

(Haven't restarted to Opera 70 by now; don't wanna know ... ).

@guenterw: The solution provided in https://forums.opera.com/post/195641 solved the problem. (At least for now ....?)

@robinjmoffatt: Oh, great. That worked! Many thanks.

I was very happy with "Bookmarks by the Side" until the update to Opera 66 a few days ago. I used "Bookmarks by the Side" as a very thin column, only showing the web site's icons (like it was possible with early opera installations). This doesn't work any longer. Now there is a minimum width of about 1/5 of the screen on my 14'-Laptop. - Please do reset this minimum width!