Ok - a couple of hours of working on this later:
The bug is that ST sets a fixed pixel height to the body element, but does not handle the standard zoom setting at all.
I have set "Options -> Settings -> Standard Zoom" to "Far". (Have to guess a little concerning the names, my Android is German.)
ST only works correctly if this is set to "Medium".
If I set it to "Close", then only the top few pixels of the toolbar are visible at all.
Why does ST need to set a fixed height? If I override Ext.viewport.Default.setHeight to always use 100%, then my app seems to behaves fine (at first sight).
This is down the to autoMaximize method in Ext.Viewport which tries to hide the address bar when it exists.
In the next release we have decided to change the default value of this to false, instead of true. There are just too many situations where there are issues with the viewport not expanding. Not to mention the fact that the performance is slightly worst when it is enabled.
If you ever want to enable it in your application int he future, you can use the following configuration in Ext.setup or Ext.application: