@sgunhouse
Your question prompted me to consider my Opera installation. Disabling extensions had no effect, but it made me think about the fact that all my post-Opera 12 installations were being installed in a single directory dating back to Opera 32, so I installed Opera 39 in a "Opera39" directory and now I'm mangle-free.
Going forward I'll install new major versions in version-specific folders.
Version 40? Yes, because there's an ongoing unresolved severe memory leak issue reported for versions after Opera 40. I've noticed a mild leakage problem with the older versions I'm using where Opera will consume substantially larger amounts of RAM when left open for several days. This is probably an issue with the Chromium engine since Chrome also gobbles RAM as well if left open.
Given Opera's quirkiness, this mangled rendering issue being a prime example, I'm holding off on a fully current update until I see a resolution.
Thanks for the input.