It ran in an 80's monthly sci-fi/Fantasy magazine, either Isaac Asimov's Fantasy and Science Fiction, or Analog.
The main character was a guy who was rummaging through an old house he either bought or inherited, and found a flashlight. The flashlight seemed normal at first, but then he noticed the beam allowed him to look into alternate realities. He walked around the house, getting glimpses of a woman he became infatuated with, but something was wrong. This woman seemed to be living in the same house (in a state of disrepair), but it was under siege by these feline humanoids with "claws like ten-penny nails". The man would have left the light on, but he could see no way to replace the batteries and it was dying...and occasionally, the creatures he observed would turn toward him and squint as if they could see the flashlight's beam.
He later saw the woman under full attack by the creatures ("leapers", I think he dubbed them, because of their muscular legs and jumping ability), and they displayed a new trick: they could slash thin air with those "ten-penny" claws (the author sure loved that description, as he used it a LOT) and tear through space to teleport into the house, where the woman, now badly injured, had barricaded herself into the bedroom in a last stand. The guy never saw her last moments because the flashlight finally died, but not before it flickered on again and showed the creatures gathered around her body, feasting...then one turned towards him and again squinted in the beam's glare.
It's implied that the cat-monsters homed in on the light's source, and were slashing their way into his dimension in the last couple sentences. Good story, hope someone can help me find it or the author's name. Maybe it's been reprinted in some sci-fi anthology.