Hi, I've also been having this issue on and off for the last few months. Thought it got fixed something like a month ago but now it got to the point where I have to force quit / restart Opera every 2 hours or so becomes closing tabs becomes ridiculously exhausting
Can't reproduce it consistently, only thing I've noticed is that opening a webapp like Google Hangouts (so memory-heavy I presume) almost always starts the problem, but it's been happening without me opening a Hangouts tab too. Sticky thumbnail also happening systematically when the double-click to close bug happens.
Pleeeease Opera team look into this soon