Filled polygons: You have to go into the style menu and select fill style, by default all filled shapes are filled by white, guess it is confusing as white is bg default colour too, but yeah, filled does work, select your shape in Tool>Go to new tab named Style>Fill>Select your style and apply
Resolution limit: because vibes? I think Decker being low res is one of its very cool selling points, maybe a technical limitation, but I love decker’s fixed resolution!