Don't ask the user for a screen configuration on login, instead, apply a sane default.
When connecting a new screen, apply a default configuration so the user gets immediate feedback "something's happening" and then prompt for a configuration via the OSD in case the chosen default doesn't fit.

BUG: 398816

The login case is especially annoying as it would usually choose a sane default and then the user would choose Cancel which won't save a config and then kscreen asks again next time until the user choses anyhting but Cancel.

Nuked my kscreen config, restarted kded, two screens attached, had kscreen apply a default config for my two screens without asking.
Nuked my kscreen config, unplugged one monitor, restarted kded, plugged in second screen, KScreen applied a sane default (though it cannot know that the screens are swapped), and showed the OSD which I then used to swap the monitors "extend left" to how they should be