Zooming with TileManager does not work with GeoExt 1.x and webkit browsers

While testing the latest OL 2.13-rc1 release with GeoExt, I have discovered that when zooming to any previously loaded extent/zoom the TileManager will fail to load any tiles. As a side-effect, the mousewheel zoom control becomes disabled. Tiles seem to return temporarily after using the +/- zoom control which also re-enables the mousewheel zoom control.

Affecting webkit flavours: Chrome, Chromium & Safari. 

To reproduce with basic GeoExt mappanel example:
- Modify http://api.geoext.org/1.1/examples/mappanel-div.html
- point to a local 2.13-rc1 build of OpenLayers 
- enable the TileManager:
```
    var map = new OpenLayers.Map({
        "tileManager": new OpenLayers.TileManager()
    });
```
- zoom in once
- zoom out once