Fix EBO ordering and context-loss handling in GL viewport

setIndexSet could run before the scene's buffers were generated (ebo == 0),
causing GL_INVALID_OPERATION "buffer not bound". Now the index set is retained
and applied only once the scene is uploaded, re-applied after a fresh upload,
and GL handles are dropped/re-queued on context (re)init.

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

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

@@ -30,6 +30,9 @@ public class GLViewport {
     // state set off-thread, consumed in render callback
     private final AtomicReference<SceneData> pendingScene = new AtomicReference<>();
     private final AtomicReference<int[]> pendingIndexSet = new AtomicReference<>();
+    // last index set seen; retained so it can be (re)applied after the scene is uploaded or the GL
+    // context is recreated (the index buffer only exists once the scene's buffers have been generated)
+    private volatile int[] lastIndices;
     private volatile int selectedId = 0;
     private volatile boolean wireframe = false;
     private volatile boolean cull = false;
@@ -53,7 +56,15 @@ public class GLViewport {
 
     public GLViewport() {
         canvas = new GLCanvas(new JOGLExecutor(), GLProfile.CORE);
-        canvas.addOnInitEvent(ev -> initialized = false);
+        canvas.addOnInitEvent(ev -> {
+            // GL context (re)created: shaders/buffers from any previous context are gone. Drop our
+            // handles and re-queue the current scene so it is re-uploaded into the new context.
+            initialized = false;
+            scene.markContextLost();
+            if (currentScene != null) {
+                pendingScene.set(currentScene);
+            }
+        });
         canvas.addOnRenderEvent(ev -> render((JOGLRenderEvent) ev));
         canvas.addOnReshapeEvent(ev -> { /* aspect handled per-frame from width/height */ });
         installInputHandlers();
@@ -110,13 +121,20 @@ public class GLViewport {
         }
 
         SceneData newScene = pendingScene.getAndSet(null);
+        boolean uploadedNow = false;
         if (newScene != null) {
             scene.upload(gl, newScene);
             currentScene = newScene;
+            uploadedNow = true;
         }
         int[] newIndices = pendingIndexSet.getAndSet(null);
         if (newIndices != null) {
-            scene.setIndexSet(gl, newIndices);
+            lastIndices = newIndices;
+        }
+        // Apply the index set only once the scene's buffers exist (the element buffer is created by
+        // upload). Re-apply after a fresh upload so the indices survive a scene swap or context loss.
+        if (scene.hasData() && lastIndices != null && (newIndices != null || uploadedNow)) {
+            scene.setIndexSet(gl, lastIndices);
         }
         if (highlightsDirty) {
             overlay.setPoints(gl, hlPoints, hlPointColors);

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

@@ -58,6 +58,10 @@ public class GlScene {
 
     /** Re-uploads which triangles to draw (element indices). Cheap; called on filter/LOD changes. */
     public void setIndexSet(GL3 gl, int[] indices) {
+        if (!uploaded || vao == 0) {
+            // buffers not generated yet; the caller re-applies once the scene is uploaded
+            return;
+        }
         gl.glBindVertexArray(vao);
         gl.glBindBuffer(GL.GL_ELEMENT_ARRAY_BUFFER, ebo);
         IntBuffer ib = toIntBuffer(indices);
@@ -78,6 +82,16 @@ public class GlScene {
 
     public boolean hasData() { return uploaded; }
 
+    /**
+     * Forgets GL handles after the context was lost/recreated, without issuing GL calls (the old
+     * handles belong to a dead context). The next {@link #upload} regenerates everything.
+     */
+    public void markContextLost() {
+        vao = 0;
+        indexCount = 0;
+        uploaded = false;
+    }
+
     public void dispose(GL3 gl) {
         if (vao != 0) {
             gl.glDeleteVertexArrays(1, new int[]{vao}, 0);