Add z-index support for UI layers and improve render order

Introduced a set_layer_z_index method to UISystem to allow explicit control of layer stacking order. Updated create_desktop_ui to assign z-index values to desktop layers for correct visual hierarchy. Modified get_layers_for_rendering to sort layers by z-index, ensuring proper rendering from background to foreground.

Author: tristanpoland

crates/prisma_compositor/src/main.rs

@@ -79,12 +79,18 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
 async fn create_desktop_ui(compositor: &Arc<Compositor>) -> Result<(), Box<dyn std::error::Error>> {
     let ui_system = compositor.window_manager.read().unwrap().get_ui_system();
 
-    // Create desktop layers
+    // Create desktop layers with proper z-ordering
     let wallpaper_layer = ui_system.create_layer("Wallpaper".to_string());
     let desktop_layer = ui_system.create_layer("Desktop".to_string());
     let windows_layer = ui_system.create_layer("Windows".to_string());
     let taskbar_layer = ui_system.create_layer("Taskbar".to_string());
 
+    // Set z-index for proper layering (lower = background, higher = foreground)
+    ui_system.set_layer_z_index(wallpaper_layer, -100);  // Background
+    ui_system.set_layer_z_index(desktop_layer, -50);     // Desktop icons
+    ui_system.set_layer_z_index(windows_layer, 0);       // Windows
+    ui_system.set_layer_z_index(taskbar_layer, 100);     // Always on top
+
     // Create wallpaper
     let wallpaper = UIElement::rect(
         1,

crates/prisma_compositor/src/ui.rs

@@ -290,6 +290,15 @@ impl UISystem {
         layer_id
     }
 
+    /// Set the z-index of a layer
+    pub fn set_layer_z_index(&self, layer_id: u64, z_index: i32) {
+        if let Ok(mut layers) = self.layers.write() {
+            if let Some(layer) = layers.get_mut(&layer_id) {
+                layer.z_index = z_index;
+            }
+        }
+    }
+
     /// Add element to a layer
     pub fn add_element_to_layer(&self, layer_id: u64, element: UIElement) {
         if let Ok(mut layers) = self.layers.write() {
@@ -377,17 +386,22 @@ impl UISystem {
         }
     }
 
-    /// Get layers in render order
+    /// Get layers in render order (sorted by z_index)
     pub fn get_layers_for_rendering(&self) -> Vec<UILayer> {
         let layers = self.layers.read().unwrap();
         let layer_order = self.layer_order.read().unwrap();
 
-        layer_order
+        let mut render_layers: Vec<UILayer> = layer_order
             .iter()
             .filter_map(|&id| layers.get(&id))
             .filter(|layer| layer.visible)
             .cloned()
-            .collect()
+            .collect();
+
+        // Sort layers by z_index to ensure proper rendering order
+        // Lower z_index renders first (background), higher z_index renders last (foreground)
+        render_layers.sort_by_key(|layer| layer.z_index);
+        render_layers
     }
 
     /// Update screen size