I have had the same problem for months - I have reset the browser and settings which fixed it for only a short while. To give you an example of how annoying this is on Facebook the top (blue bar) with notifications, account settings etc. is completely off-screen. Normal web browsing is also ruined by the hit & miss nature of selecting the required menu option/ link. Selecting text is even harder! I have browsed the same sites with Safari no problem but do not want to ditch Opera as also use it sychronised with my mobile. I am using Opera version 56.0.3051.52 Mac Seirra 10.12.6