@leopik2
Setting -> advanced setting -> system -> turn off "Use hardware acceleration when available".

@leopik2
Setting -> advanced setting -> system -> turn off "Use hardware acceleration when available".

@darthagnon
I am install Windows 10 1809 and reinstall Opera, and nomore blue line on the top.

[Solve]
Blue line on the top:
I am install Windows 10 1809 and reinstall Opera, and nomore blue line on the top.
Texterea:
Setting -> advanced setting -> system -> turn off "Use hardware acceleration when available".

@picard
I found a way to fix in my Opera:
Setting -> advanced setting -> system -> turn off "Use hardware acceleration when available".

@leocg
When I install version 55. Not have bug with maximize windows, not have bug with textarea, but version 56 is auto install after restart opera. Version 56 seem like not stable.
https://imgur.com/a/YlIe8Hl

@darthagnon Me too, but no one have a way to fix it .

@picard Me too, I think Opera 65 not stable.

@leocg
No, only Opera, and now I found a bug with texterea.
Image bug top blue line: https://imgur.com/a/FtetHPL
Image bug text erea: https://imgur.com/a/i5GV2vB

After I updated my Opera brower, I see a blue line on the top opera window like when i'm NOT use maxize, but it's only on the top.
I have read thread: https://forums.opera.com/topic/29032/after-today-s-update-maximized-window-has-weird-blue-line but it's not working with me.
I update windows to newest version but nothing change. I don't wanna see this line anymore.

Sorry for my English. Thanks for reading!