Popup menus using PopupPanel has no default border like PopupMenu's have.

[Giganzo]

Tested versions

Godot v4.4.dev1

System information

Fedora Linux 40 (KDE Plasma) - Wayland - Vulkan (Forward+)

Issue description

Popup menus using PopupPanel has no default border like PopupMenu's have.

This makes it hard to differentiate from the background:

Compared to Godot 3:

Steps to reproduce

Open popup menus like dock position, screen selector or color picker

Minimal reproduction project (MRP)

N/A