Can a supernova or a close passing star create a transient sun gravity focus "habitable zone"?
Supernovas have a light curve going from normal to peak and back on the order of a month. Hence the spot would go from too cold to habitable and back very quickly.
The same is true for stars passing the focal point: their motion (typically tens of km per second) makes the heated point move equally fast.
Habitable zone denotes a region where a planet could have liquid water (and maybe other benign environmental properties). This implicitly assumes that this is a stable state, since the context is planetary orbits and the emergence of life, a process that takes long time. A ultra-brief habitable zone is missing the spirit of the concept, even if it may be correct to state that a certain location is habitable over a short span of time.
As the deadly focal point post discusses, the brightening is also not enormous: to get a habitable spot, you need a fairly nearby supernova or star, so the gain is not enormous.