paint_text_field used to paint an unconditional white fill, a default
grey border and a 6px radius floor, with near-black value text — a model
embedding an input in its own styled wrapper zeroes all of that out
(fill:[], stroke thickness 0, cornerRadius 0) and got a glaring white
pill with a duplicate icon on a dark themed search bar. No authored fill
now paints no box, no authored stroke draws no border, radius is taken
as-is, and the value text color adapts to the authored fill's luminance.
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