The flat twin shares the geometry, not the material

Two renderings of one control — one drawing an expensive decorative surface, one drawing flat theme surfaces — are a single form in two materials. The setting is allowed to change what the pixels are made of. It is not allowed to change where anything is.

// adapted — the flat twin's constants, named to say what they mirror
// Mirrors the decorative bar's geometry (TabWidth / BarHeight / BlobHeight / BarInset) so the two
// bars are one form in two materials.
private val FlatTabWidth = 96.dp
private val FlatBarHeight = 64.dp
private val FlatIndicatorHeight = 56.dp
private val CapsuleInset = 6.dp