Thanks for the response @Gwen-Dragon. Apologies for the tardy reply; I completely forgot about this thread!
Good news: I'm still using Vivaldi and loving it (as a developer too).
I reckon the issue was that I was viewing the page at a slightly different zoom level. Bear in mind that both the dev tools frame and the page itself have separate zoom levels, meaning that if there's an accidental minimal zoom change in play, it may not be visually obvious, and only show as a discrepancy in the dev tools resolutions.
Te resolve, click into each separately and select Page Zoom Reset from the View menu (or whatever key combo you have set up for it).
So, I reckon it was a fairly naive user error at fault here, rather than a bug with the software, as originally suggested.
Hopefully that sorts it for anyone else that stumbles upon the same - somewhat embarrassing - issue.