@LonM Yeah, that's what I was suspecting. This extension (https://chrome.google.com/webstore/detail/scroll-preview/hhpfdnhklmhheldplcmfhfdooeekgpmm?hl=en), for example, gets around this problem by auto-hiding the minimap when the mouse cursor is nowhere near the right edge of the screen. I, personally, think that those are cheap workarounds; both, translucent minimap and auto-hiding ones. I think, being a browser feature and not just an extension, the minimap implementation in Vivaldi should replace the scrollbar, resize/shorten/narrow'en the page size to accomodate the minimap, and be always there (i.e., no auto-hiding) instead.
Speaking of narrow window/page sizes, (forced) word-wrap should be a thing too; perhaps a 'page-action'... Some article-style sites aren't very responsive to the window size, and require a horizontal scrollbar at the bottom... Nevermind.