fix: `BrowserWindow.moveAbove()` not working for child windows (#39034)

fix: BrowserWindow.moveAbove() not working for child windows

shell/browser/native_window_mac.mm

@@ -162,6 +162,47 @@ void ViewDidMoveToSuperview(NSView* self, SEL _cmd) {
   [self setFrame:[[self superview] bounds]];
 }
 
+// -[NSWindow orderWindow] does not handle reordering for children
+// windows. Their order is fixed to the attachment order (the last attached
+// window is on the top). Therefore, work around it by re-parenting in our
+// desired order.
+void ReorderChildWindowAbove(NSWindow* child_window, NSWindow* other_window) {
+  NSWindow* parent = [child_window parentWindow];
+  DCHECK(parent);
+
+  // `ordered_children` sorts children windows back to front.
+  NSArray<NSWindow*>* children = [[child_window parentWindow] childWindows];
+  std::vector<std::pair<NSInteger, NSWindow*>> ordered_children;
+  for (NSWindow* child in children)
+    ordered_children.push_back({[child orderedIndex], child});
+  std::sort(ordered_children.begin(), ordered_children.end(), std::greater<>());
+
+  // If `other_window` is nullptr, place `child_window` in front of
+  // all other children windows.
+  if (other_window == nullptr)
+    other_window = ordered_children.back().second;
+
+  if (child_window == other_window)
+    return;
+
+  for (NSWindow* child in children)
+    [parent removeChildWindow:child];
+
+  const bool relative_to_parent = parent == other_window;
+  if (relative_to_parent)
+    [parent addChildWindow:child_window ordered:NSWindowAbove];
+
+  // Re-parent children windows in the desired order.
+  for (auto [ordered_index, child] : ordered_children) {
+    if (child != child_window && child != other_window) {
+      [parent addChildWindow:child ordered:NSWindowAbove];
+    } else if (child == other_window && !relative_to_parent) {
+      [parent addChildWindow:other_window ordered:NSWindowAbove];
+      [parent addChildWindow:child_window ordered:NSWindowAbove];
+    }
+  }
+}
+
 }  // namespace
 
 NativeWindowMac::NativeWindowMac(const gin_helper::Dictionary& options,
@@ -759,13 +800,21 @@ bool NativeWindowMac::MoveAbove(const std::string& sourceId) {
   if (!webrtc::GetWindowOwnerPid(window_id))
     return false;
 
-  [window_ orderWindow:NSWindowAbove relativeTo:id.id];
+  if (!parent()) {
+    [window_ orderWindow:NSWindowAbove relativeTo:window_id];
+  } else {
+    NSWindow* other_window = [NSApp windowWithWindowNumber:window_id];
+    ReorderChildWindowAbove(window_, other_window);
+  }
 
   return true;
 }
 
 void NativeWindowMac::MoveTop() {
-  [window_ orderWindow:NSWindowAbove relativeTo:0];
+  if (!parent())
+    [window_ orderWindow:NSWindowAbove relativeTo:0];
+  else
+    ReorderChildWindowAbove(window_, nullptr);
 }
 
 void NativeWindowMac::SetResizable(bool resizable) {