The two content halves sized from their natural widths, so the light
half ran 7px past the calc(21px + 50%) background and drew a dark
step and hairline at the seam. A homogeneous box now shares the space
equally. GTK does not inherit text direction, so the side card and
accent dot still flipped in RTL; every part is pinned LTR. The GTK
test asserts both.