Same issue here.
From my observations, it is caused by Opera pop-up options for Video pop-out and Lucid mode options at the top of video window. They never really hide when they roll back. In standard mode they are cover top portion of video with hard to notice lines, but remain at the same spot for full screen mode. Hence, when you turn Lucid mode one (Opera browser), the longer line turns blueish, which can be really annoying.
Apparently, you can try to fix it with turning of hardware acceleration (doesn´t work for me). From what I found, it can affect more likely devices with older hardware, Windows 10 and Linux OS. It has been addressed before, but never fixed. The same problem is discussed in this forum.
I think there is a chance, that Lucid mode will arrive to GX too, so it is the right time to address this issue.
My specs: Win 10, 6th Intel U-processor, integrated graphics