Fix recursion panic when PopupWindow.show() called in init callback (#9498)

- Defer user_init() call using event loop in both Rust and C++ backends
- Add fallback to synchronous call when event loop unavailable
- Fixes issue where init callbacks calling popup.show() caused infinite recursion

Changes:
- internal/compiler/generator/rust.rs: Defer user_init via invoke_from_event_loop
- api/cpp/include/slint_window.h: Defer user_init via slint_post_event
- Applied to both ShowPopupWindow and ShowPopupMenu cases

This change breaks the recursion by deferring the user_init execution,
allowing the popup to be properly initialized before init callbacks run.

Author: Rishant12220055

api/cpp/include/slint_window.h

@@ -93,7 +93,15 @@ class WindowAdapterRc
         auto popup_dyn = popup.into_dyn();
         auto id = cbindgen_private::slint_windowrc_show_popup(&inner, &popup_dyn, p, close_policy,
                                                               &parent_item, false);
-        popup->user_init();
+        // Defer user_init to prevent recursion when init callbacks call popup.show()
+        cbindgen_private::slint_post_event(
+            [](void *data) { 
+                auto popup = *reinterpret_cast<decltype(popup) *>(data);
+                popup->user_init(); 
+            }, 
+            new decltype(popup)(popup),
+            [](void *data) { delete reinterpret_cast<decltype(popup) *>(data); }
+        );
         return id;
     }
 
@@ -123,7 +131,15 @@ class WindowAdapterRc
         auto id = cbindgen_private::slint_windowrc_show_popup(
                 &inner, &popup_dyn, pos, cbindgen_private::PopupClosePolicy::CloseOnClickOutside,
                 &context_menu_rc, true);
-        popup->user_init();
+        // Defer user_init to prevent recursion when init callbacks call popup.show()
+        cbindgen_private::slint_post_event(
+            [](void *data) { 
+                auto popup = *reinterpret_cast<decltype(popup) *>(data);
+                popup->user_init(); 
+            }, 
+            new decltype(popup)(popup),
+            [](void *data) { delete reinterpret_cast<decltype(popup) *>(data); }
+        );
         return id;
     }
 

internal/compiler/generator/rust.rs

@@ -2912,7 +2912,14 @@ fn compile_builtin_function_call(
                             false, // is_menu
                         ))
                     );
-                    #popup_window_id::user_init(popup_instance_vrc.clone());
+                    // Defer user_init to prevent recursion when init callbacks call popup.show()
+                    let popup_instance_vrc_for_init = popup_instance_vrc.clone();
+                    if let Err(_) = i_slint_core::api::invoke_from_event_loop(move || {
+                        #popup_window_id::user_init(popup_instance_vrc_for_init);
+                    }) {
+                        // Fallback: if event loop is not available (e.g., during testing), call synchronously
+                        #popup_window_id::user_init(popup_instance_vrc.clone());
+                    }
                 })
             } else {
                 panic!("internal error: invalid args to ShowPopupWindow {arguments:?}")
@@ -2991,7 +2998,14 @@ fn compile_builtin_function_call(
                     true, // is_menu
                 );
                 #set_id;
-                #popup_id::user_init(popup_instance_vrc);
+                // Defer user_init to prevent recursion when init callbacks call popup.show()
+                let popup_instance_vrc_for_init = popup_instance_vrc.clone();
+                if let Err(_) = i_slint_core::api::invoke_from_event_loop(move || {
+                    #popup_id::user_init(popup_instance_vrc_for_init);
+                }) {
+                    // Fallback: if event loop is not available (e.g., during testing), call synchronously
+                    #popup_id::user_init(popup_instance_vrc);
+                }
             };
 
             let common_init = quote! {