It appears on the sidebar but it cannot be disabled

Hi after newest update in the left corner a chrome-like strange window appears, what is that?
When that window appears opera also creates a process that takes ~8% of CPU!

Someone already described it here:
https://forums.opera.com/topic/44934/unknown-icon-in-sidebar

I use Opera for Linux

I have the same issue, when the sidebar is disabled it appears in the left corner, what is that?