I don't know how accessibility like zoom feature have appeared. Maybe I have input some keys' combination. I don't know how to get rid of it. I've tried to disable it in:
Start menu -> Settings Manager -> Settings Editor -> xfwm4
unchecking zoom_desktop
value, but this frame have not disappeared.
Here it is:
Today I decided to record some screen area using vokoscreen. The same frame appeared.
It seems likely the frame in my question have been produced somehow by a vokoscreen without a vokoscreen being launched by me. Looks like it was a bug.
So I could get rid of it by
vokoscreen
launching and by switching to Fullscreen as I think.