Systems, devices, and methods for preventing eyebox degradation in wearable heads-up displays (“WHUDs”) are described. A WHUD may provide an eyebox defined by multiple spatially-separated exit pupils, where the size of the eyebox is influenced by the spacing between the exit pupils. A larger eyebox is achieved with larger spacing between the exit pupils but a larger spacing between exit pupils is susceptible to the formation of gaps or blind spots in the eyebox (eyebox degradation) if bright environmental light causes the user's pupil to constrict to a size that is smaller than the spacing between the exit pupils. A material of variable chromism is included in the user's field of view to reduce the perceived brightness of environmental light and thereby prevent the user's pupil from constricting to a size smaller than the spacing between the exit pupils.