  We report resonant elastic X-ray scattering (REXS) of long-range magnetic
order in EuPtSi$_{\text{3}}$, combining different scattering geometries with
full linear polarization analysis to unambiguously identify magnetic scattering
contributions. At low temperatures, EuPtSi$_{\text{3}}$ stabilizes type A
antiferromagnetism featuring various long-wavelength modulations. For magnetic
fields applied in the hard magnetic basal plane, well-defined regimes of
cycloidal, conical, and fan-like superstructures may be distinguished that
encompass a pocket of commensurate type A order without superstructure. For
magnetic field applied along the easy axis, the phase diagram comprises the
cycloidal and conical superstructures only. Highlighting the power of polarized
REXS, our results reveal a combination of magnetic phases that suggest a highly
unusual competition between antiferromagnetic exchange interactions with
Dzyaloshinsky--Moriya spin--orbit coupling of similar strength.
