refactor: make get_popup_toplevel() PopupKind agnostic

- for preparation to make unconstrain_popup() PopupKind agnostic.

src/shell/layout/floating/mod.rs

@@ -1278,7 +1278,7 @@ impl FloatingLayout {
         let Some(focused) = (match target {
             KeyboardFocusTarget::Popup(popup) => {
                 let Some(toplevel_surface) = (match popup {
-                    PopupKind::Xdg(xdg) => get_popup_toplevel(&xdg),
+                    PopupKind::Xdg(_) => get_popup_toplevel(&popup),
                     PopupKind::InputMethod(_) => unreachable!(),
                 }) else {
                     return MoveResult::None;