macOS: fall back to the current event in BeginMoveDrag when no mouse-down was recorded (#21827)

BeginMoveDrag on macOS drags from the Avalonia view's last recorded
mouse-down. A press that begins inside an embedded native view (for
example a webview hosted through NativeControlHost) is consumed by that
view and never delivered to the Avalonia view, so lastMouseDownEvent is
nil and BeginMoveDrag silently does nothing - applications embedding
native views cannot implement custom title-bar dragging through the
managed API and must reproduce performWindowDragWithEvent: through
platform interop.

When no mouse-down was recorded, fall back to the event the application
is currently tracking, guarded to left-button press or drag events that
belong to this window. Presses recorded by the Avalonia view behave
exactly as before.

native/Avalonia.Native/src/OSX/WindowBaseImpl.mm

@@ -347,6 +347,21 @@ HRESULT WindowBaseImpl::BeginMoveDrag() {
     @autoreleasepool {
         auto lastEvent = [View lastMouseDownEvent];
 
+        if (lastEvent == nullptr) {
+            // A press that begins inside an embedded native view (for example a webview hosted
+            // through NativeControlHost) is consumed by that view and never delivered to the
+            // Avalonia view, so no mouse-down is recorded; fall back to the event the
+            // application is tracking right now, provided it is a left-button press or drag
+            // that belongs to this window.
+            auto currentEvent = [NSApp currentEvent];
+
+            if (currentEvent != nullptr && [currentEvent window] == Window &&
+                ([currentEvent type] == NSEventTypeLeftMouseDown ||
+                 [currentEvent type] == NSEventTypeLeftMouseDragged)) {
+                lastEvent = currentEvent;
+            }
+        }
+
         if (lastEvent == nullptr) {
             return S_OK;
         }