  Thin and thick disks are found in most spiral galaxies, yet their formation
scenarios remain uncertain. Whether thick disks form through slow or fast,
internal or environmental, processes is unclear. The physical origin of outer
truncations in thin and thick disks, observed as a drop in optical and
near-infrared (NIR) surface brightness profiles, is also a much debated topic.
These truncations have been linked to star formation (SF) thresholds in
Milky-Way type galaxies, but no such connection has been made for their
low-mass counterparts or in thick disks. Our photometric analysis of the
edge-on galaxy UGC 7321 offers a possible breakthrough. This well-studied
diffuse, isolated, bulgeless, ultra-thin galaxy is thought to be under-evolved
both dynamically and in SF. It is an ideal target to disentangle internal
effects in the formation of thick disks and truncations. Our axial light
profiles from deep far- and near-ultraviolet (UV; GALEX) images, tracing recent
SF, and optical (DESI grz) and NIR (Spitzer 3.6 microns) images, tracing old
stellar populations, enable a detailed identification of an outer truncation in
all probed wavelengths in both the thin and thick disks. After deprojecting to
a face-on view, a sharp truncation signature is found at a stellar density of
roughly 1.5 solar masses per square parsec, in agreement with theoretical
expectations of gas density SF thresholds. The redder colours beyond the
truncation radius are indicative of stellar migration towards the outer
regions. We thus show that thick disks and truncations can form via internal
mechanisms alone, given the pristine nature of UGC 7321. We report the
discovery of a truncation at and above the mid-plane of a diffuse galaxy that
is linked to a SF threshold; this poses a constraint on physically-motivated
disk size measurements among low-mass galaxies.
