The board wears a picture — background becomes a mapping, and the window chrome follows it under a thin frost

background is {color:, image:} and only a mapping at every level; the board's image paints the full window under a transparent title bar, with a thin-material frost strip keeping the chrome legible and the standard accommodations intact.

Claude-Session: https://claude.ai/code/session_014PtZdPwqZuqEDLc6wZMtEy

Kanban/UI/Accommodations.swift

@@ -90,19 +90,28 @@ enum Accommodations {
     /// appear").
     ///
     /// The design's own example (the card face carousel's page dots) died with the carousel
-    /// (03-board-ui.md § Card face's no-carousel resettlement), so the rule's one surviving subject
-    /// on the board is the transient search bar's `.bar` material. It is stated as a type anyway
-    /// rather than inlined at that one call site, because "wherever they appear" is a standing rule
-    /// and the next material to arrive should find the answer already written.
+    /// (03-board-ui.md § Card face's no-carousel resettlement), so the rule's subjects on the board
+    /// are the transient search bar's `.bar` material and the backdrop's title-bar frost — the
+    /// "next material to arrive" this type was stated for, and it found the answer already written.
     enum Underlay: Equatable {
         /// `Material.bar` — the find-bar's own backdrop, translucent over the board beneath it.
         case glass
+        /// `Material.thin` — the title-bar frost over a custom board backdrop
+        /// (`BoardView.boardBackground`). Deliberately not `.bar` — the find-bar sits over lanes
+        /// the board's own plates have already calmed, where the frost sits directly on an image
+        /// the author may well have chosen *for* its busyness — and deliberately not the heavier
+        /// notches either, tried and retired: `.ultraThick` read as a cloudy plate where a backdrop
+        /// should still show through, and `.regular` still veiled it more than the chrome needs.
+        /// The thin weight carries the chrome, and the dissolve below it (`BoardView.frostStrip`)
+        /// is what keeps the strip from reading as a bar.
+        case frost
         /// The window's own background colour, opaque.
         case solid
 
         var style: AnyShapeStyle {
             switch self {
             case .glass: AnyShapeStyle(.bar)
+            case .frost: AnyShapeStyle(.thinMaterial)
             case .solid: AnyShapeStyle(Color(nsColor: .windowBackgroundColor))
             }
         }
@@ -112,6 +121,12 @@ enum Accommodations {
         reduceTransparency ? .solid : .glass
     }
 
+    /// The title-bar frost's own reading of the same rule — heavier glass, identical accommodation:
+    /// under Reduce Transparency both underlays take the one solid.
+    static func frost(reduceTransparency: Bool) -> Underlay {
+        reduceTransparency ? .solid : .frost
+    }
+
     /// A **translucent wash** — a tint laid over whatever happens to be behind it, and the shape
     /// every non-material translucency on the board takes: every lane's plate, the trash column's
     /// plate and hatched header, and the drag shadow's fill.