extjs - FieldContainer makes the form overflow -

i using extjs 5. tree panel placed in form.because want label tree,so put tree field container put field container form.

...... {   xtype:'form',   items:[{     xtype:'fieldcontainer',     label:'tree',     items:[{         xtype:'treepanel',         root:{             expanded:false,             children:[{...}]//this content big enough make tree overflow.         }     }]   ] } ...... 

i know fix height config can solve it.but don't want give fix height/maxheight tree or container.i want fulfil whole space in browser. can help?