I put your panel code in a west region in the code below and it seemed to work ok for me in 4.2.1.
Code:
Ext.create('Ext.panel.Panel', {
width: 500,
height: 300,
title: 'Border Layout',
layout: 'border',
items: [{
title: 'South Region is resizable',
region: 'south', // position for region
xtype: 'panel',
height: 100,
split: true, // enable resizing
margins: '0 5 5 5'
},{
title: 'West',
border: false,
region: 'west',
split: true,
width: 250,
buttonAlign: 'right',
unstyled: true,
fbar: {
xtype: 'toolbar',
hideBorders: true,
items: [{
id: 'action.submit',
text: 'Submit',
handler: function () {
return imSubmitAction('action.submit', '0', true);
}
}, {
id: 'action.cancel',
text: 'Cancel',
cls: 'ca-gray',
handler: function () {
imSubmitAction('action.cancel', '1', false);
}
}
]
}
},{
title: 'Center Region',
region: 'center', // center region is required, no width/height specified
xtype: 'panel',
layout: 'fit',
margins: '5 5 0 0'
}],
renderTo: Ext.getBody()
});