In Dialog.conditionalShow() there's the following logic:

if (modal) {
  ...
  modalShow();
}
...
peer.setVisible(true);

Then:

modalShow() -> blockWindows() -> peer.blockWindows() -> LWWindowPeer.setModalBlocked() -> CPlatformWindow.setModalBlocked() -> checkBlockingAndOrder() ->

        CWrapper.NSWindow.orderFrontRegardless(nsWindowPtr);
        CWrapper.NSWindow.makeKeyAndOrderFront(nsWindowPtr);
        CWrapper.NSWindow.makeMainWindow(nsWindowPtr);

This actually shows the platform window, before peer.setVisible(true) is called.

This seems unexpected and is a potential source of issues (like 8160570).