Hi Igor!
I'm also having the same problem, especially on google docs and what not.
As another mac user thats been using Opera for a bit now, the only thing that I can recommend you to do is:

• not be in full screen mode
  If you know what im saying, go ahead and just ignore this.
  Basically what I would do is enlarge opera to fit the screen to its maximum BUT not be in full screen mode.
  That way, when you press ESC button especially on websites like google docs/sheets, it won't become a small window.
  I also find that this is quite annoying, so another thing I would also do is have 2 'desktops' open if you know what I mean, so you can switch to the other desktop to open other apps like finder and the like.
  I know that it's not the best solution, but I hope that it helps a bit.

Solution.
On macbooks, if you look on the google shortcuts (cmd+/)
and if you press cmd+f to search for shift+v
you find that the paste without formatting is set as...

cmd+option+v
However.

If You Press.
cmd+option+shift+v
It will work.

unfortunately there is no way as far as I know to edit the shortcuts on google docs because...
well because it sucks.
however at least I found the super annoying but working way of pasting without formatting (and clicking right click and clicking left click again every single time)

it is very annoying i agree.

Hi.
This is what's happening to me right now:
I play a video, and the pop up will show up.
Then, I return to the video and go back to my work page or whatnot. Then it is not automatically poping up.
I've found that sometimes reloading or closing/reopening works but haven't found a solution (yet)

Hi Igor!
I'm also having the same problem, especially on google docs and what not.
As another mac user thats been using Opera for a bit now, the only thing that I can recommend you to do is:

• not be in full screen mode
  If you know what im saying, go ahead and just ignore this.
  Basically what I would do is enlarge opera to fit the screen to its maximum BUT not be in full screen mode.
  That way, when you press ESC button especially on websites like google docs/sheets, it won't become a small window.
  I also find that this is quite annoying, so another thing I would also do is have 2 'desktops' open if you know what I mean, so you can switch to the other desktop to open other apps like finder and the like.
  I know that it's not the best solution, but I hope that it helps a bit.

Solution.
On macbooks, if you look on the google shortcuts (cmd+/)
and if you press cmd+f to search for shift+v
you find that the paste without formatting is set as...

cmd+option+v
However.

If You Press.
cmd+option+shift+v
It will work.

unfortunately there is no way as far as I know to edit the shortcuts on google docs because...
well because it sucks.
however at least I found the super annoying but working way of pasting without formatting (and clicking right click and clicking left click again every single time)