  We use deep Hubble Space Telescope imaging to study the resolved stellar
populations in BST1047+1156, a gas-rich, ultradiffuse dwarf galaxy found in the
intragroup environment of the Leo I galaxy group. While our imaging reaches
approximately two magnitudes below the tip of the red giant branch at the Leo I
distance of 11 Mpc, we find no evidence for an old red giant sequence that
would signal an extended star formation history for the object. Instead, we
clearly detect the red and blue helium burning sequences of its stellar
populations, as well as the fainter blue main sequence, all indicative of a
recent burst of star formation having taken place over the past 50--250 Myr.
Comparing to isochrones for young metal-poor stellar populations, we infer this
post-starburst population to be moderately metal poor, with metallicity [M/H]
in the range -1 to -1.5. The combination of a young, moderately metal-poor post
starburst population and no old stars motivates a scenario in which BST1047 was
recently formed during a weak burst of star formation in gas that was tidally
stripped from the outskirts of the neighboring massive spiral M96. BST1047's
extremely diffuse nature, lack of ongoing star formation, and disturbed HI
morphology all argue that it is a transitory object, a "failing tidal dwarf" in
the process of being disrupted by interactions within the Leo I group. Finally,
in the environment surrounding BST1047, our imaging also reveals the old,
metal-poor ([M/H]=-1.3 +/- 0.2) stellar halo of M96 at a projected radius of 50
kpc.
