Improve handling of temporary view focus switch in mousemove.

Closes #841

src/dom/DomElement.js

@@ -84,14 +84,14 @@ var DomElement = new function() {
         },
 
         /**
-         * Checks if element is invisibile (display: none, ...)
+         * Checks if element is invisibile (display: none, ...).
          */
         isInvisible: function(el) {
             return DomElement.getSize(el).equals(new Size(0, 0));
         },
 
         /**
-         * Checks if element is visibile in current viewport
+         * Checks if element is visibile in current viewport.
          */
         isInView: function(el) {
             // See if the viewport bounds intersect with the windows rectangle
@@ -101,6 +101,13 @@ var DomElement = new function() {
                         DomElement.getBounds(el, true));
         },
 
+        /**
+         * Checks if element is inside the DOM.
+         */
+        isInserted: function(el) {
+            return document.body.contains(el);
+        },
+
         /**
          * Gets the given property from the element, trying out all browser
          * prefix variants.

src/view/View.js

@@ -407,12 +407,21 @@ var View = Base.extend(Emitter, /** @lends View# */{
      * Checks whether the view is currently visible within the current browser
      * viewport.
      *
-     * @return {Boolean} whether the view is visible
+     * @return {Boolean} {@true if the view is visible}
      */
     isVisible: function() {
         return DomElement.isInView(this._element);
     },
 
+    /**
+     * Checks whether the view is inserted into the browser DOM.
+     *
+     * @return {Boolean}  {@true if the view is inserted}
+     */
+    isInserted: function() {
+        return DomElement.isInserted(this._element);
+    },
+
     /**
      * Scrolls the view by the given vector.
      *
@@ -799,12 +808,16 @@ new function() { // Injection scope for mouse events on the browser
                     // Key events are handled too during the mouse over.
                     // As we switch view, fire one last mousemove in the old
                     // view, to let items receive receive a mouseleave, etc.
-                    handleMouseMove(view, event);
+                    if (view)
+                        handleMouseMove(view, event);
                     prevFocus = view;
                     view = View._focused = tempFocus = target;
                 }
             } else if (tempFocus && tempFocus === view) {
-                // Clear temporary focus again and update it.
+                // Clear temporary focus again and switch back to previous focus
+                // but only if it is still valid (still in the DOM).
+                if (prevFocus && !prevFocus.isInserted())
+                    prevFocus = null;
                 view = View._focused = prevFocus;
                 updateFocus();
             }