Put the mainform QWidget inside of a QMainWindow box, so things work out a bit better.

src/main.ui

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <ui version="4.0">
  <class>MainWindow</class>
- <widget class="QWidget" name="mainform">
+ <widget class="QMainWindow" name="MainWindow">
   <property name="geometry">
    <rect>
     <x>0</x>
@@ -25,6 +25,7 @@
   <property name="windowTitle">
    <string>Form</string>
   </property>
+  <widget class="QWidget" name="mainform">
    <layout class="QGridLayout" name="gridLayout">
     <item row="6" column="0">
      <layout class="QVBoxLayout" name="specs">
@@ -421,6 +422,7 @@
     </item>
    </layout>
   </widget>
+ </widget>
  <resources>
   <include location="main.qrc"/>
  </resources>