You probably right about "weird" fonts, I checked and saw "dingobats" font installed, those icons look pretty much like that font. Probably Opera not scaling it properly.

Just tried to grey out messages behind blue ones in different orders and looks like I need click just one "right" message for it to turn grey. When I tried it earlier it appeared like I need click all of them because most likely "right" one was the last one I clicked
I see now in screenshot that gray hand have one bottom message grayed - I sure this is message it belongs to.

Mystery is solved

Checked older messages and looks like more appears at various spots in all areas, my oldest messages in this client are from June. I have 4 email addresses, all gmail, widgets are spreading over all accounts. Here some more from older messages, and no, they not over spam messages only, I just picking spam messages for screenshots instead of personal ones
Also they changing to grey color only if all subjects behind them are grayed out - as long as at lest one stays blue, widget is blue too.

This is one from July, so they definitely season related.

They stay in the same spot and changing colors with text colors - when I click on messages so they grayed out, widgets turn to grey color too. If Im scrolling they kinda breaking, but restoring as soon I click on dif app or on any of messages. Closing all other tabs and restating browser not affects it.

I know its really old browser, but I use it mainly as mail client. Recently I moved it to new Focal Fossa 64 bit system, (entire folder with settings) and now I can see some weird widgets in mail client on top of messages list. Its snowman today, but there been old style alarm clock and bell I think. I never seen it on older 32 bit Linux, no other browser have it anywhere. It not bothers me much, just like to know where it coming from.

image url)