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/History/BoardStoreHistory.swift

@@ -345,17 +345,21 @@ extension BoardStore {
     /// the write — or removes the key, which is what "before" means for a field that was not there.
     ///
     /// A **malformed** prior reads as a removal, and that is the one place an inverse is not
-    /// byte-exact: the app cannot re-emit `background: [a, b]` through a document edit that only
-    /// knows how to set scalars. It is also the case the forward write was designed to clear
-    /// ("choosing any well replaces it" — 03-board-ui.md § Styling ▸ Controls), so the undo lands the
-    /// item on the app's own reading of that field rather than resurrecting a value nothing could
-    /// read.
+    /// byte-exact: the app cannot re-emit `background: [a, b]` — or a hand-written scalar
+    /// `background: green`, which the schema stopped reading when the key became a mapping — through
+    /// a document edit that only writes the shapes the schema names. It is also exactly the case the
+    /// forward write was designed to clear ("choosing any well replaces it" — 03-board-ui.md §
+    /// Styling ▸ Controls), so the undo lands the item on the app's own reading of that field rather
+    /// than resurrecting a value nothing could read.
+    ///
+    /// The **mapping** case needs no branch of its own here and gets none: `setStyleValue` edits the
+    /// `color` subkey and leaves the rest (BackgroundField.swift), so an undo on a board with an
+    /// image restores the colour the board had — including restoring it to *absent* — without
+    /// disturbing the image the forward write already preserved. What the inverse does not promise
+    /// is the author's subkey order when the colour was absent before: a restored colour that had no
+    /// pair to go back to is appended, like any newly written subkey.
     static func restore(_ prior: FieldValue<String>, to key: String, in document: inout FrontmatterDocument) {
-        if let value = prior.value {
-            document.set(key, to: .string(value))
-        } else {
-            document.remove(key)
-        }
+        document.setStyleValue(prior.value, for: key)
     }
 
     /// The style fields a gesture actually set — **one entry per dimension it did not `.keep`**, so a