Shared helper for rail popovers that host the custom GUI.
A rail popover (see ControlRailPopover) is itself the glass surface;
the GUI mounted inside it should read as flat content. This helper creates a
GUI parented to the popover host, strips its own glass marker (so the
liquid-glass ::before/::after layers don't double up), and shows it. The
popover CSS neutralizes the GUI's fixed positioning / frame.
Shared helper for rail popovers that host the custom GUI.
A rail popover (see ControlRailPopover) is itself the glass surface; the GUI mounted inside it should read as flat content. This helper creates a GUI parented to the popover host, strips its own glass marker (so the liquid-glass ::before/::after layers don't double up), and shows it. The popover CSS neutralizes the GUI's fixed positioning / frame.