Guarantee redraw delivery for on-demand GL rendering

openglfx's requestRepaint is an unconditional notify with no pending flag, so a
repaint() arriving while a frame is mid-render is dropped. When a scene was
built, the setScene repaint rendered an index-less (empty) frame and the
following setIndexSet repaint was lost, so the scene only appeared after the
next rotate/zoom. Route all repaints through requestRender(), which re-delivers
via a small AnimationTimer pump until a frame consumes the request, then stops
itself (idle stays allocation-free).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CityDoctorParent/Extensions/CityDoctorGUI/src/main/java/de/hft/stuttgart/citydoctor2/gui/gl/GLViewport.java

@@ -8,6 +8,7 @@ import com.jogamp.opengl.GL;
 import com.jogamp.opengl.GL3;
 import java.util.concurrent.atomic.AtomicReference;
 import java.util.function.Consumer;
+import javafx.animation.AnimationTimer;
 import javafx.application.Platform;
 import javafx.scene.input.MouseButton;
 import javafx.scene.paint.Color;
@@ -55,6 +56,22 @@ public class GLViewport {
     private double lastX;
     private double lastY;
 
+    // on-demand render request. openglfx can drop a repaint() that arrives while a frame is rendering
+    // (its requestRepaint is an unconditional notify with no pending flag), so a single repaint is not
+    // reliable. This flag + pump re-deliver repaint() each pulse until a frame consumes the request,
+    // then the pump stops itself so an idle viewport has no per-pulse cost.
+    private volatile boolean renderRequested;
+    private final AnimationTimer repaintPump = new AnimationTimer() {
+        @Override
+        public void handle(long now) {
+            if (renderRequested) {
+                canvas.repaint();
+            } else {
+                stop();
+            }
+        }
+    };
+
     public GLViewport() {
         // args: executor, profile, flipY=false, msaa=4, fps=0.0
         // fps=0 -> on-demand rendering: a frame is drawn only when repaint() is called (every state
@@ -72,8 +89,8 @@ public class GLViewport {
         canvas.addOnRenderEvent(ev -> render((JOGLRenderEvent) ev));
         canvas.addOnReshapeEvent(ev -> { /* aspect handled per-frame from width/height */ });
         // on-demand mode: repaint when the canvas is resized so the framebuffer tracks the new size
-        canvas.widthProperty().addListener((obs, ov, nv) -> canvas.repaint());
-        canvas.heightProperty().addListener((obs, ov, nv) -> canvas.repaint());
+        canvas.widthProperty().addListener((obs, ov, nv) -> requestRender());
+        canvas.heightProperty().addListener((obs, ov, nv) -> requestRender());
         installInputHandlers();
     }
 
@@ -81,15 +98,15 @@ public class GLViewport {
 
     // ---- public API (called from FX / worker threads) ----
 
-    public void setScene(SceneData data) { pendingScene.set(data); canvas.repaint(); }
+    public void setScene(SceneData data) { pendingScene.set(data); requestRender(); }
 
-    public void setIndexSet(int[] indices) { pendingIndexSet.set(indices); canvas.repaint(); }
+    public void setIndexSet(int[] indices) { pendingIndexSet.set(indices); requestRender(); }
 
-    public void setWireframe(boolean on) { wireframe = on; canvas.repaint(); }
+    public void setWireframe(boolean on) { wireframe = on; requestRender(); }
 
-    public void setCulling(boolean on) { cull = on; canvas.repaint(); }
+    public void setCulling(boolean on) { cull = on; requestRender(); }
 
-    public void setPointScale(float scale) { pointScale = scale; canvas.repaint(); }
+    public void setPointScale(float scale) { pointScale = scale; requestRender(); }
 
     /** points: xyz, pointColors rgb-per-point; lines: xyz pairs, lineColors rgb-per-line-vertex. */
     public void setHighlights(float[] points, float[] pointColors, float[] lines, float[] lineColors) {
@@ -98,20 +115,32 @@ public class GLViewport {
         this.hlLines = lines;
         this.hlLineColors = lineColors;
         this.highlightsDirty = true;
-        canvas.repaint();
+        requestRender();
     }
 
     public OrbitCamera camera() { return camera; }
 
+    /**
+     * Requests a redraw. Sets the render flag, wakes the canvas immediately (best effort), and starts
+     * the pump so the request is re-delivered until a frame actually consumes it. Call on the FX thread.
+     */
+    private void requestRender() {
+        renderRequested = true;
+        canvas.repaint();
+        repaintPump.start(); // idempotent while running
+    }
+
     public void requestPick(double x, double y, Consumer<Object> callback) {
         pickRequest = new double[]{x, y};
         pickCallback = callback;
-        canvas.repaint();
+        requestRender();
     }
 
     // ---- render callback (GL thread) ----
 
     private void render(JOGLRenderEvent ev) {
+        // consume the render request; if new state arrives during this frame, requestRender re-arms it
+        renderRequested = false;
         GL3 gl = ev.getGl();
         int w = ev.width;
         int h = ev.height;
@@ -221,13 +250,13 @@ public class GLViewport {
                 double speed = Math.abs(camera.distance()) / 1000.0;
                 camera.pan(-dx * speed, dy * speed);
             }
-            canvas.repaint();
+            requestRender();
         });
         canvas.setOnScroll(e -> {
             camera.zoom(e.getDeltaY() < 0 ? 1.05 : 0.95);
-            canvas.repaint();
+            requestRender();
         });
     }
 
-    public void resetCamera() { camera.reset(); canvas.repaint(); }
+    public void resetCamera() { camera.reset(); requestRender(); }
 }