Refactor WebcamPreviewPage to avoid duplicate updates

Co-authored-by: toniolo.luca <toniolo.luca@outlook.com>

Author: cursoragent

src/Captura/Pages/WebcamPlacementPreviewPage.xaml.cs

@@ -108,33 +108,39 @@ void OnRegionChange()
 
             PreviewGrid.LayoutUpdated += (S, E) => OnRegionChange();
 
-            _webcamModel
-                .ObserveProperty(M => M.SelectedCam)
-                .Subscribe(M => UpdateWebcamPreview());
-
-            _reactor.Location
-                .CombineLatest(_reactor.Size, (M, N) =>
-                {
-                    UpdateWebcamPreview();
-                    return 0;
-                })
-                .Subscribe();
-
             // If page is already visible when setup is first called, initialize webcam
             // (IsVisibleChanged won't fire if page is already visible)
+            var needsInitialUpdate = false;
             if (IsVisible && _webcamCapture == null)
             {
                 _webcamCapture = _webcamModel.InitCapture();
 
                 if (_webcamCapture.Value is { } capture)
                 {
                     _reactor.WebcamSize.OnNext(new WSize(capture.Width, capture.Height));
-                    // UpdateWebcamPreview will be called by the Location/Size subscription
+                    needsInitialUpdate = true;
                 }
             }
-            else
+
+            // Set up reactive subscriptions after initial webcam setup
+            // Skip first emission to avoid duplicate update during initialization
+            _reactor.Location
+                .CombineLatest(_reactor.Size, (M, N) =>
+                {
+                    UpdateWebcamPreview();
+                    return 0;
+                })
+                .Skip(needsInitialUpdate ? 1 : 0)
+                .Subscribe();
+
+            _webcamModel
+                .ObserveProperty(M => M.SelectedCam)
+                .Skip(1) // Skip initial value to avoid duplicate update
+                .Subscribe(M => UpdateWebcamPreview());
+
+            // Only call update once if we initialized the webcam during setup
+            if (needsInitialUpdate)
             {
-                // Just update preview position if webcam already exists
                 UpdateWebcamPreview();
             }
         }