Restricts horizontal scrolling to mouse over (#3953)

Ensures horizontal mouse wheel events only affect the ScrollViewer when the mouse cursor is directly over it. This prevents unintended scrolling of background or non-focused elements.

Simplifies the scroll handling by inlining the logic and removing a redundant helper method.

Author: Keboo

src/MaterialDesignThemes.Wpf/ScrollViewerAssist.cs

@@ -140,18 +140,13 @@ IntPtr Hook(IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam, ref bool handled
                 const int WM_MOUSEHWHEEL = 0x020E;
                 switch (msg)
                 {
-                    case WM_MOUSEHWHEEL:
+                    case WM_MOUSEHWHEEL when scrollViewer.IsMouseOver:
                         int tilt = (short)((wParam.ToInt64() >> 16) & 0xFFFF);
-                        OnMouseTilt(tilt);
+                        scrollViewer.ScrollToHorizontalOffset(scrollViewer.HorizontalOffset + tilt);
                         return (IntPtr)1;
                 }
                 return IntPtr.Zero;
             }
-
-            void OnMouseTilt(int tilt)
-            {
-                scrollViewer.ScrollToHorizontalOffset(scrollViewer.HorizontalOffset + tilt);
-            }
         }
     }