macos: "Check for updates" cancels whatever the current update state is (#9203)

This mainly allows users who have a pending update but didn't install it
for some time to re-check to see if there is something newer in the mean
time.

macos/Sources/Features/Update/UpdateController.swift

@@ -93,7 +93,22 @@ class UpdateController {
     ///
     /// This is typically connected to a menu item action.
     @objc func checkForUpdates() {
-        updater.checkForUpdates()
+        // If we're already idle, then just check for updates immediately.
+        if viewModel.state == .idle {
+            updater.checkForUpdates()
+            return
+        }
+        
+        // If we're not idle then we need to cancel any prior state.
+        installCancellable?.cancel()
+        viewModel.state.cancel()
+        
+        // The above will take time to settle, so we delay the check for some time.
+        // The 100ms is arbitrary and I'd rather not, but we have to wait more than
+        // one loop tick it seems.
+        DispatchQueue.main.asyncAfter(deadline: .now() + .milliseconds(100)) { [weak self] in
+            self?.updater.checkForUpdates()
+        }
     }
     
     /// Validate the check for updates menu item.