Things I tried:

• all possible Vertical policy's without success.
• Change the FieldGrowthPolicy of the layout to AllNonFixedFieldsGrow
• Use another widget, all fail to make this work

That's all the options I found to possibly make this work.

Here's what I try to achieve:

Here's my ui file:

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>Dialog</class>
 <widget class="QDialog" name="Dialog">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>760</width>
    <height>747</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>Dialog</string>
  </property>
  <layout class="QFormLayout" name="formLayout">
   <item row="0" column="0">
    <widget class="QLabel" name="label">
     <property name="text">
      <string>Please grow the treeview:</string>
     </property>
    </widget>
   </item>
   <item row="0" column="1">
    <widget class="QTreeWidget" name="treeWidget">
     <property name="sizePolicy">
      <sizepolicy hsizetype="Expanding" vsizetype="MinimumExpanding">
       <horstretch>0</horstretch>
       <verstretch>0</verstretch>
      </sizepolicy>
     </property>
     <column>
      <property name="text">
       <string notr="true">1</string>
      </property>
     </column>
    </widget>
   </item>
  </layout>
 </widget>
 <resources/>
 <connections/>
</ui>