initialize InputManager.cursorPos to fix issue #792 for LWJGL2

jme3-core/src/main/java/com/jme3/input/InputManager.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2009-2012 jMonkeyEngine
+ * Copyright (c) 2009-2018 jMonkeyEngine
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -423,15 +423,22 @@ public class InputManager implements RawInputListener {
 
     /**
      * Callback from RawInputListener. Do not use.
+     *
+     * @param evt event to add to the input queue (not null)
      */
     @Override
     public void onMouseMotionEvent(MouseMotionEvent evt) {
-        if (!eventsPermitted) {
-            throw new UnsupportedOperationException("MouseInput has raised an event at an illegal time.");
-        }
-
+        /*
+         * If events aren't allowed, the event may be a "first mouse event"
+         * triggered by the constructor setting the the mouse listener.
+         * In that case, use the event to initialize the cursor position,
+         * but don't queue it for futher processing.
+         * This is part of the fix for issue #792.
+         */
         cursorPos.set(evt.getX(), evt.getY());
-        inputQueue.add(evt);
+        if (eventsPermitted) {
+            inputQueue.add(evt);
+        }
     }
 
     private void onMouseButtonEventQueued(MouseButtonEvent evt) {

jme3-lwjgl/src/main/java/com/jme3/input/lwjgl/LwjglMouseInput.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2009-2012 jMonkeyEngine
+ * Copyright (c) 2009-2018 jMonkeyEngine
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -85,6 +85,10 @@ public class LwjglMouseInput implements MouseInput {
         } catch (LWJGLException ex) {
             logger.log(Level.SEVERE, "Error while creating mouse", ex);
         }
+
+        if (listener != null) {
+            sendFirstMouseEvent();
+        }
     }
 
     public boolean isInitialized(){
@@ -158,8 +162,32 @@ public class LwjglMouseInput implements MouseInput {
         Mouse.setGrabbed(!visible);
     }
 
+    @Override
     public void setInputListener(RawInputListener listener) {
         this.listener = listener;
+        if (listener != null && Mouse.isCreated()) {
+            sendFirstMouseEvent();
+        }
+    }
+
+    /**
+     * Send the input listener a special mouse-motion event with zero deltas in
+     * order to initialize the listener's cursor position.
+     */
+    private void sendFirstMouseEvent() {
+        assert listener != null;
+        assert Mouse.isCreated();
+
+        int x = Mouse.getX();
+        int y = Mouse.getY();
+        int xDelta = 0;
+        int yDelta = 0;
+        int wheelDelta = 0;
+        MouseMotionEvent evt = new MouseMotionEvent(x, y, xDelta, yDelta,
+                curWheel, wheelDelta);
+        evt.setTime(Mouse.getEventNanoseconds());
+
+        listener.onMouseMotionEvent(evt);
     }
 
     public long getInputTimeNanos() {