Fix pill visibility: always render, use opacity to hide

NSHostingView with frame (0,0) won't layout SwiftUI content, so
fittingSize always returns 0 when transitioning from EmptyView.
Instead, always render the pill button and use opacity/frame(0)
to hide when idle. This ensures the hosting view always has content
to measure.

Sources/Update/UpdatePill.swift

@@ -11,17 +11,18 @@ struct UpdatePill: View {
 
     var body: some View {
         let state = model.effectiveState
-        if !state.isIdle {
-            pillButton
-                .popover(
-                    isPresented: $showPopover,
-                    attachmentAnchor: .rect(.bounds),
-                    arrowEdge: .top
-                ) {
-                    UpdatePopoverView(model: model)
-                }
-                .transition(.opacity.combined(with: .scale(scale: 0.95)))
-        }
+        let visible = !state.isIdle
+        pillButton
+            .popover(
+                isPresented: $showPopover,
+                attachmentAnchor: .rect(.bounds),
+                arrowEdge: .top
+            ) {
+                UpdatePopoverView(model: model)
+            }
+            .opacity(visible ? 1 : 0)
+            .frame(width: visible ? nil : 0, height: visible ? nil : 0)
+            .clipped()
     }
 
     @ViewBuilder