Fix menu placement coordinate adjustments on macOS

Corrects the logic for menu positioning based on placement, ensuring the menu appears at the intended location relative to the reference point. Adds clarifying comments and updates coordinate calculations for all placement cases.

Author: lijy91

src/platform/macos/menu_macos.mm

@@ -703,59 +703,69 @@ - (void)menuDidClose:(NSMenu*)menu {
   double menu_height = menu_size.height;
 
   // Adjust position based on placement
+  // Note: Coordinates are in top-left origin system (y grows downward)
+  // popUpMenuPositioningItem places the menu's top-left corner at the specified location
   switch (placement) {
-    case Placement::TopStart:  // topLeft
-      x -= menu_width;
-      y += menu_height;
+    case Placement::TopStart:  // Menu above reference point, left-aligned
+      // Menu's bottom-left corner at reference point
+      // No x adjustment needed (left-aligned)
+      // Move up by menu height
+      y -= menu_height;
       break;
 
-    case Placement::Top:  // top center
+    case Placement::Top:  // Menu above reference point, center-aligned
+      // Menu's bottom-center at reference point
       x -= menu_width / 2.0;
-      y += menu_height;
+      y -= menu_height;
       break;
 
-    case Placement::TopEnd:  // topRight
-      y += menu_height;
+    case Placement::TopEnd:  // Menu above reference point, right-aligned
+      // Menu's bottom-right corner at reference point
+      x -= menu_width;
+      y -= menu_height;
       break;
 
-    case Placement::RightStart:  // right top
-      x += menu_width;
-      y += menu_height;
+    case Placement::RightStart:  // Menu to the right, top-aligned
+      // Menu's top-left corner at reference point (no adjustment needed)
       break;
 
-    case Placement::Right:  // right center
-      x += menu_width;
+    case Placement::Right:  // Menu to the right, center-aligned
+      // Menu's left-center at reference point
       y -= menu_height / 2.0;
       break;
 
-    case Placement::RightEnd:  // right bottom
-      x += menu_width;
+    case Placement::RightEnd:  // Menu to the right, bottom-aligned
+      // Menu's bottom-left corner at reference point
       y -= menu_height;
       break;
 
-    case Placement::BottomStart:  // bottomLeft
-      x -= menu_width;
+    case Placement::BottomStart:  // Menu below reference point, left-aligned
+      // Menu's top-left corner at reference point (no adjustment needed)
       break;
 
-    case Placement::Bottom:  // bottom center
+    case Placement::Bottom:  // Menu below reference point, center-aligned
+      // Menu's top-center at reference point
       x -= menu_width / 2.0;
       break;
 
-    case Placement::BottomEnd:  // bottomRight
-      // No adjustment needed
+    case Placement::BottomEnd:  // Menu below reference point, right-aligned
+      // Menu's top-right corner at reference point
+      x -= menu_width;
       break;
 
-    case Placement::LeftStart:  // left top
+    case Placement::LeftStart:  // Menu to the left, top-aligned
+      // Menu's top-right corner at reference point
       x -= menu_width;
-      y += menu_height;
       break;
 
-    case Placement::Left:  // left center
+    case Placement::Left:  // Menu to the left, center-aligned
+      // Menu's right-center at reference point
       x -= menu_width;
       y -= menu_height / 2.0;
       break;
 
-    case Placement::LeftEnd:  // left bottom
+    case Placement::LeftEnd:  // Menu to the left, bottom-aligned
+      // Menu's bottom-right corner at reference point
       x -= menu_width;
       y -= menu_height;
       break;