  The $\mathcal{CP}$ structure of the Higgs boson in its coupling to the
particles of the Standard Model is amongst the most important Higgs boson
properties which have not yet been constrained with high precision. In this
study, all relevant inclusive and differential Higgs boson measurements from
the ATLAS and CMS experiments are used to constrain the $\mathcal{CP}$-nature
of the top-Yukawa interaction. The model dependence of the constraints is
studied by successively allowing for new physics contributions to the couplings
of the Higgs boson to massive vector bosons, to photons, and to gluons. In the
most general case, we find that the current data still permits a significant
$\mathcal{CP}$-odd component in the top-Yukawa coupling. Furthermore, we
explore the prospects to further constrain the $\mathcal{CP}$ properties of
this coupling with future LHC data by determining $tH$ production rates
independently from possible accompanying variations of the $t\bar t H$ rate.
This is achieved via a careful selection of discriminating observables. At the
HL-LHC, we find that evidence for $tH$ production at the Standard Model rate
can be achieved in the Higgs to diphoton decay channel alone.
