I have tried to disable screen blacking/screen saver in KDE 5:

System settings ($ systemsettings5) / Workspace Behavior / Screen locking, set to lock after 15 minutes
System settings ($ systemsettings5) / Power management / Energy saving / "Dim screen", "Screen energy saving", "Suspend session" - all are not ticked (disabled)

yet after a few minutes screen gets blank and after moving mouse, sometimes the parts of the active app window interface, like buttons start flickering and for example web page in web browser is not scroll-able until i click for example kick-off menu and close the menu, then it refresh page, then i switch other browser tab and click menu again to reflect changes. Restart browser, issue is gone, until next screen saver.

xscreensaver is not installed "locate kscreensaverrc" - nothing.
Someone suggest to turn off screen blanking (for current session?) by command "xset s off" yet i think that if that is necessary, then KDE should be fixed so it allows user to disable it via GUI in System settings above.

How to disable it? I have not reboot some days.

$ inxi -Sazy

System:
Kernel: 5.15.0-0.bpo.3-amd64 x86_64 bits: 64 compiler: gcc v: 10.2.1
parameters: BOOT_IMAGE=/vmlinuz-5.15.0-0.bpo.3-amd64
root=/dev/mapper/ab--vg-root ro quiet
Desktop: KDE Plasma 5.20.5 tk: Qt 5.15.2 wm: kwin_x11 dm: SDDM
Distro: Debian GNU/Linux 11 (bullseye)

$ xset -q

...
Screen Saver:
prefer blanking: yes allow exposures: yes
timeout: 120 cycle: 600
...
DPMS (Energy Star):
Standby: 0 Suspend: 0 Off: 0
DPMS is Enabled
Monitor is On