Okay, I have a workaround, and I've tested on Dell laptop (where I experienced the problem), and on my two desktops ... all of them running openSuse Tumbleweed distro, and the KDE Plasma desktop environment.
Please note: this fix applies to KDE Plasma D.E., so if you're running GNOME or other D.E., this will probably not work.
a) Start Opera and resize its window to your preference, and location on the screen.
b) Use the keyboard combination <Alt> + <F3> ... on my Dell laptop, I have to use <Alt><Fn> + <F3>.
c) See screenshot #1 ... choose More Actions, and Configure Special Window Settings.
d) See screenshot #2 ... Add the three Properties shown - be sure they are set to "Remember".
e) Click Apply and OK
See screenshot #3 ... you will now see an "extra" topbar, which by default is not shown
KDE border menu ... to More Settings
KDE Add Properties
Resulting "extra" KDE topbar