  We report results from far-ultraviolet observations by the Hubble Space
Telescope of Jupiter's largest moon Ganymede transiting across the planet's
dayside hemisphere. {Within} a targeted campaign on 9 September 2021 two
exposures were taken during one transit passage to probe for attenuation of
Jupiter's hydrogen Lyman-$\alpha$ dayglow above the moon limb. The background
dayglow is slightly attenuated over an extended region around Ganymede, with
stronger attenuation in the second exposure when Ganymede was near the planet's
center. In the first exposure when the moon was closer to Jupiter's limb, the
effects from the Ganymede corona are hardly detectable, likely because the
Jovian Lyman-$\alpha$ dayglow is spectrally broader and less intense at this
viewing geometry. The obtained vertical H column densities of around
$(1-2)\times 10^{12}$~cm$^{-2}$ are consistent with previous results.
Constraining angular variability around Ganymede's disk, we derive an upper
limit on a local H$_2$O column density of $(2-3)\times 10^{16}$~cm$^{-2}$, such
as could arise from outgassing plumes in regions near the observed moon limb.
