I'm very happy to ascertain the fact the problem has been solved in the version 53.0.2907.88!!! Good job Opera!!! Bravo!

@burnout426 How would you interpret the colloquialism "the bug is fixed internally"???
As far as I got from the composition the bug had been fixed for a short-listed circle of developers and common users still have no certain roadmap or even a hint when the stable release will bring an updated stability (and/or will it ever bring?)... Shall we have to consider the only solution for the moment is the turnin' the Win 8 compatibility on???

@leocg further experiments revealed the fact the problem exists only on Windows 10 Redstone 4 1803 version... That's a bug the Opera developers' team have not completely adapted the release to the latest most actual version of Windows 10...

В папке [resources] был какой-то файл... название типо ............partner........json (точно не вспомню). Он и добавлял сайты рекламодателей текущих сборок... Если это так... то одноразовым удалением этого json-a вопрос не решить... Прилетит новое обновление с новой [resources] папкой, а там снова эти партнёрские json-ы... Тогда возможным (не уверен, не пробовал, просто в качестве предположения) решением может быть → обновил+не запустил, в [ресурсах] удалил json, потом только запускать оперу...
*(Перед удалением любого файла из любой папки любого ПО советую создать его резервную копию)

Launching the Opera 53.0.2907.68 on 64-bit MS Windows 10 April update (no other versions of Opera and Windows were tested) causes the following bug:
First time maximizing the window does it normally, but the next repeat gives a result: when the window is maximized it leaves a little gap above, and the next maximize does it normally again...
Please, try to reproduce the issue yourself, offer solutions, comment...
Thanks in advance for your attention.
And here's a loooong video which demonstrates the bug)))