Success! Looks like we've fixed this one. According to our records the fix was applied for
a recent build.
Ext.device.Notification.Simulator.show disappears under Ext.Panel (z-index problem)
If you call Ext.MessageBox.show directly you can use the zIndex option. However, if you use Ext.device.Notification.show - which relays to Ext.device.notification.Simulator when not packed as native app - you cannot and the z-index always has the default value of 10.
If you call Ext.device.Notification.show from a (floating) Ext.Panel and/or after some Ext.NavigationView.push() call's, the z-index will be lower then the Ext.Panel and the dialog will disappear under the Ext.Panel and will be invisible and unclickable.
Probably this is not a huge problem since you'll only call Ext.MessageBox.show through Ext.device.Notification.show if you're releasing the app as a native app, which will use Ext.device.Notification.Sencha or PhoneGap anyway.
Ext.device.Notification.Simulator could find the highest z-index and pass it (+ 1) to Ext.device.Notification.show, or just pass 9999 all the time
Thank you for the report.
Sencha - Sencha Touch Dev Team
Fixed for the next release. Thank you!