'hiddenfield' occupies the visible place in the form. EXTJSIV-6081 comes back in extjs 4.2.0. Even height is set to 0.
See the test code. Tested with FF and Chrome.

PHP Code:
Ext.onReady(function() {

    var 
form Ext.create('Ext.form.Panel', {
    
layout'anchor',
        
borderfalse,
        
defaultType'textfield',
        
bodyPadding5,
        
items: [{
            
fieldLabel'Send To',
            
name'to',
            
anchor:'100%' 
        
},{
            
fieldLabel'Subject',
            
name'subject',
            
anchor'100%' 
        
}, {
            
xtype'hiddenfield',
            
name'h1',
        
height0,
        
value'h1'
        
}, {
            
xtype'hiddenfield',
            
name'h2',
        
height0,
        
value'h2'
        
}, {
            
xtype'hiddenfield',
            
name'h3',
        
height0,
        
value'h3'
        
}, {
            
xtype'hiddenfield',
            
name'h4',
        
height0,
        
value'h4'
        
}, {
            
xtype'hiddenfield',
            
name'h5',
        
height0,
        
value'h5'
        
}, {
            
xtype'hiddenfield',
            
name'h6',
        
height0,
        
value'h6'
        
}, {
            
xtype'hiddenfield',
            
name'h7',
        
height0,
        
value'h7'
        
}, {
            
xtype'hiddenfield',
            
name'h8',
        
height0,
        
value'h8'
        
}, {
            
xtype'hiddenfield',
            
name'h9',
        
height0,
        
value'h9'
        
}, {
            
xtype'hiddenfield',
            
name'h10',
        
height0,
        
value'h10'
        
}, {
            
xtype'hiddenfield',
            
name'h7',
        
value'h7'
        
}, {
            
xtype'textarea',
            
hideLabeltrue,
        
minHeight100,
            
name'msg',
            
anchor'100% -67' 
        
}]
    });
    var 
win Ext.create('Ext.window.Window', {
        
title'Resize Me',
        
width500,
        
height:600,
        
layout'fit',
    
maximizabletrue,
        
plaintrue,
        
itemsform,
    
listeners: { resize: function() { } },
        
buttons: [{
            
text'Send'
        
},{
            
text'Cancel'
        
}]
    });
    
win.show();

});