I can enter STOP mode and by pulsing pa0 I can sorta-kinda recover from it, the shell no longer works but the blinker thread resumes. All this does not work if WFI_IDLE is enabled for some reason...

Standby mode can be entered (at least I think it's standby, based on the ~2uA current consumption...) but can only be recovered from by reset (either I do not bind the pa0 interrupt correctly or something else goes wrong, it doesn't even come to bootloader [this would be another problem since pa0 and boot0 are connected])