The black stripes are back in version 48.0.2679.0, but just in the chat windows from sidebar.
Composition seens doesn't have effect over it.
Using KDE 5.10 on Manjaro Linux x64.
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Same problem here! Doesn't matter if system effects are disabled or if I use a new profile.
In fact, with system effects disabled, the problem seems worse (the black border is bigger).
Using Opera Developer 48.0.2643.0 in Manjaro x64 with KDE 5.9.