The inactive Plan/Fly button used to sit inset by the header's own
padding on all sides, leaving a visible strip of the header tint
around it instead of filling the header exactly like the HTML
prototype. CSS solves this with a negative margin, which Flutter's
Padding widget rejects (padding.isNonNegative assertion) - restructured
so only the active button stays wrapped in the inset padding, while the
inactive one is a raw Row child that naturally touches the header
container's true top/bottom/outer edges and gets clipped to its rounded
corner. Also adds a short gradient from the header's own tint into the
button's solid color at the seam, softening what used to be a hard
color edge.
Constantin Leue
853cce5b26