Ray-caster: only draw when the scene is changed

This is how I typically schedule draw calls in JavaScript applications.
The baseline CPU activity for the display prototype is now in line with
other pages (though perhaps a bit higher), and the profiler shows little
time being spent in draw calls, even when I'm continually moving a
slider. The interface feels pretty responsive overall, although the
sliders seem to be lagging a bit.

app-proto/inversive-display/src/main.rs

@@ -108,6 +108,21 @@ fn main() {
         // display
         let display = create_node_ref();
         
+        // change listener
+        let scene_changed = create_signal(true);
+        create_effect(move || {
+            ctrl_x.track();
+            ctrl_y.track();
+            radius_x.track();
+            radius_y.track();
+            opacity.track();
+            highlight.track();
+            layer_threshold.track();
+            debug_mode.track();
+            
+            scene_changed.set(true);
+        });
+        
         on_mount(move || {
             // list construction elements
             const SPHERE_MAX: usize = 12;
@@ -216,7 +231,8 @@ fn main() {
             bind_vertex_attrib(&ctx, position_index, 3, &positions);
             
             // set up a repainting routine
-            let (_, start_updating_display, _) = create_raf(move || {
+            let (_, start_animation_loop, _) = create_raf(move || {
+                if scene_changed.get() {
                     /* INSTRUMENTS */
                     // measure frame time
                     frames_since_last_sample += 1;
@@ -267,15 +283,15 @@ fn main() {
                     
                     // draw the scene
                     ctx.draw_arrays(WebGl2RenderingContext::TRIANGLES, 0, VERTEX_CNT as i32);
-            });
                     
-            /*
+                    // clear scene change flag
-            this wastes CPU time by running an animation loop, which updates the
+                    scene_changed.set(false);
-            display even when nothing has changed. there should be a way to make
+                } else {
-            Sycamore do single-frame updates in response to changes, but i
+                    frames_since_last_sample = 0;
-            haven't found it yet
+                    frame_time.set(-1.0);
-            */
+                }
-            start_updating_display();
+            });
+            start_animation_loop();
         });
         
         view! {