feat(xdg_popup): blur behind popups as behind windows

A translucent popup was a clear pane over whatever it opened above.
Each commit now gives the popup's tree the window blur node, sized from
the surface and masked by its alpha, so rounded corners and shadow
margins stay clear. Regular rather than optimized blur, as for layer
surfaces stacked above windows. cce-ui's context menu popup relies on
it: it paints its plate as the surface's root and leaves the frost to
the compositor. popup_server() is factored out of handle_reposition,
which needed the same lookup.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

 src/server/xdg_popup.rs | 63 +++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 53 insertions(+), 10 deletions(-)

diff --git a/src/server/xdg_popup.rs b/src/server/xdg_popup.rs
index abcb8961..8153c96c 100644
--- a/src/server/xdg_popup.rs
+++ b/src/server/xdg_popup.rs
@@ -83,6 +83,58 @@ unsafe extern "C" fn handle_commit(listener: *mut ffi::wl_listener, _data: *mut
     let base_surface = ffi::river_wlr_xdg_popup_get_base((*popup).wlr_popup);
     if ffi::river_wlr_xdg_surface_get_initial_commit(base_surface) {
         handle_reposition(&mut (*popup).reposition, std::ptr::null_mut());
+        return;
+    }
+    update_blur(popup, base_surface);
+}
+
+/// Blur behind the popup as behind a window: a translucent menu is frosted
+/// glass, and with no blur node it is a clear pane over whatever it opened
+/// above. cce-ui paints its context-menu popup as the surface's ROOT plate
+/// for exactly this — translucent, with the frost left to the compositor,
+/// since the client has no backdrop to frost from inside its own popup.
+///
+/// Masked by the surface's own alpha (`ignore_transparent`), so the menu's
+/// rounded corners and its shadow margin stay clear; never the optimized
+/// (cached) blur, which re-bakes on every change beneath a surface stacked
+/// above windows — see `handle_layer_surface_commit`. Sized from the
+/// surface, so it runs every commit: a popup is resized by its configure.
+unsafe fn update_blur(popup: *mut XdgPopup, base_surface: *mut ffi::wlr_xdg_surface) {
+    let server = popup_server(popup);
+    if server.is_null() {
+        return;
+    }
+    let wlr_surface = ffi::river_wlr_xdg_surface_get_surface(base_surface);
+    if wlr_surface.is_null() {
+        return;
+    }
+    ffi::river_scene_node_enable_blur(
+        (*popup).tree as *mut ffi::wlr_scene_node,
+        (*server).wm.layout.window_blur,
+        false,
+        (*server).wm.layout.window_backdrop_blur_ignore_transparent,
+        0,
+        0,
+        ffi::river_wlr_surface_get_width(wlr_surface),
+        ffi::river_wlr_surface_get_height(wlr_surface),
+        0,
+    );
+}
+
+/// The server a popup belongs to, found through its parent's scene node —
+/// a window or a shell surface. Null when the parent is neither.
+unsafe fn popup_server(popup: *mut XdgPopup) -> *mut crate::server::Server {
+    let parent_tree = ffi::river_wlr_scene_tree_get_parent((*popup).tree);
+    if parent_tree.is_null() {
+        return std::ptr::null_mut();
+    }
+    match crate::scene_node_data::SceneNodeData::from_node(parent_tree as *mut ffi::wlr_scene_node) {
+        Some(node_data) => match node_data.data {
+            crate::scene_node_data::SceneNodeDataVal::Window(w) => (*w).server,
+            crate::scene_node_data::SceneNodeDataVal::ShellSurface(s) => (*s).server,
+            _ => std::ptr::null_mut(),
+        },
+        None => std::ptr::null_mut(),
     }
 }
 
@@ -113,16 +165,7 @@ unsafe extern "C" fn handle_reposition(listener: *mut ffi::wl_listener, _data: *
     anchor.x += parent_lx;
     anchor.y += parent_ly;
 
-    let server = if let Some(node_data) = crate::scene_node_data::SceneNodeData::from_node(parent_tree as *mut ffi::wlr_scene_node) {
-        match node_data.data {
-            crate::scene_node_data::SceneNodeDataVal::Window(w) => (*w).server,
-            crate::scene_node_data::SceneNodeDataVal::ShellSurface(s) => (*s).server,
-            _ => std::ptr::null_mut(),
-        }
-    } else {
-        std::ptr::null_mut()
-    };
-
+    let server = popup_server(popup);
     if server.is_null() {
         return;
     }