Rebuild Displays around the canvas, and let the transaction keep color

Claude-Session: https://claude.ai/code/session_01Ms2FbjQy31TVf3CEvQhGM8

config/dot/quickshell/display-arrangement-harness.qml

@@ -75,6 +75,36 @@ ShellRoot {
             })));
         }
 
+        // One display is the common case on a laptop, and the canvas is the
+        // page's hero now: it renders that display rather than disappearing and
+        // leaving a picker for a list of one. Only dragging goes away, because
+        // there is nothing to arrange it against.
+        function soloFixture(): string {
+            const previous = fixtureService.monitors;
+            fixtureService.monitors = [previous[0]];
+            arrangement.resetDraft();
+            const snapshot = arrangement.canvasSnapshot();
+            fixtureService.monitors = previous;
+            arrangement.resetDraft();
+            return JSON.stringify(snapshot);
+        }
+
+        // A mirrored display has no position of its own -- the compositor puts
+        // it on top of its target -- so the canvas stacks it there and says so
+        // rather than drawing it wherever its stale coordinates point.
+        function mirrorFixture(): string {
+            const previous = fixtureService.monitors;
+            fixtureService.monitors = [
+                previous[0],
+                Object.assign({}, previous[1], { mirrorOf: "DP-2" })
+            ];
+            arrangement.resetDraft();
+            const snapshot = arrangement.canvasSnapshot();
+            fixtureService.monitors = previous;
+            arrangement.resetDraft();
+            return JSON.stringify(snapshot);
+        }
+
         function identify(): void { Displays.identify(); }
         function identifying(): bool { return Displays.identifying; }
     }