..In addition to previous post, following conditions are needed to trigger the issue:
Opera need to be positioned near the right edge of a monitor with high resolution (4K or QHD) with a low resolution secondary display (ordinary HD).
I tested two different systems, 4K and QHD: 3840 pixels wide and 2560 pixels wide both trigger it in a dual screen setup with a normal "HD/1920" secondary.
Opera for Linux