Fix DropDown submenus on Linux/Wayland (#13563)

Hovering a group item (e.g. "Generic" in the filament list) now opens
its submenu correctly. The dropdown stays usable while moving the
cursor between the parent list and the submenu, and dismisses normally
when the user clicks outside.

src/slic3r/GUI/Widgets/PopupWindow.hpp

@@ -17,6 +17,12 @@ public:
 #ifdef __WXMSW__
     void BindUnfocusEvent();
 #endif
+protected:
+    // Orca: Hook so derived classes (e.g. DropDown chains) can skip auto-dismissal
+    // when the toplevel deactivates as a side effect of their own popup grab
+    // (notably on Wayland, where mapping a chained xdg_popup with grab makes
+    // the parent toplevel briefly inactive).
+    virtual bool ShouldDismissOnTopWindowDeactivate() { return true; }
 private:
 #ifdef __WXOSX__
     void OnMouseEvent2(wxMouseEvent &evt);