Change the thickness of HLine/VLine in Qt Designer

When I am trying to develop a simple UI, to make a visible separation between widgets I decided to use lines. I could use them, but when I try to change the thickness of them it didn't work. And also the lines drawn are incomplete. I changed the lineWidth property to change the thickness. This is the demonstration of how I did it.

Has someone got any experience about this? Can someone show me what I am doing wrong, and how to do it properly?

_{[NB: the solution given below assumes the default Fusion widget-style is being used. Some other custom styles may impose their own settings, which could very likely produce different results]}

There are two separate issues here:

Firstly, to get the desired thickness, you must adjust the following properties of the line:

• set the frameShadow to Sunken or Raised
• set the lineWidth to zero (this is needed so as to get the exact desired thickness, since it would otherwise increase the total value)
• set the midLineWidth to the desired thickness (e.g. 10)
• set the minimumHeight (or minimumWidth, for vertical lines) to the same value as above
• [optional] set the Mid role in the palette to an appropriate colour

Secondly, to join horizontal and vertical lines so they form a T-junction, you must set the vertcal and/or horizontal spacing to zero for the layouts containing the relevant lines, and then set the stylesheet margins of the neighbouring widgets to restore the spacing wherever needed. To illustrate this, I have added below a simple Qt Designer example. This sets the vertical spacing of the main grid-layout to zero, and also sets the margin-bottom of the top widget, and the margin-top of the two bottom widgets to the default spacing of the layout:

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>Form</class>
 <widget class="QWidget" name="Form">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>400</width>
    <height>300</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>Form</string>
  </property>
  <layout class="QGridLayout" name="gridLayout">
   <property name="verticalSpacing">
    <number>0</number>
   </property>
   <item row="2" column="1">
    <widget class="Line" name="line_2">
     <property name="minimumSize">
      <size>
       <width>10</width>
       <height>0</height>
      </size>
     </property>
     <property name="lineWidth">
      <number>0</number>
     </property>
     <property name="midLineWidth">
      <number>10</number>
     </property>
     <property name="orientation">
      <enum>Qt::Vertical</enum>
     </property>
    </widget>
   </item>
   <item row="1" column="0" colspan="3">
    <widget class="Line" name="line">
     <property name="minimumSize">
      <size>
       <width>0</width>
       <height>10</height>
      </size>
     </property>
     <property name="lineWidth">
      <number>0</number>
     </property>
     <property name="midLineWidth">
      <number>10</number>
     </property>
     <property name="orientation">
      <enum>Qt::Horizontal</enum>
     </property>
    </widget>
   </item>
   <item row="2" column="0">
    <widget class="QLabel" name="label_2">
     <property name="styleSheet">
      <string notr="true">background: white; margin-top: 6px</string>
     </property>
     <property name="frameShape">
      <enum>QFrame::StyledPanel</enum>
     </property>
    </widget>
   </item>
   <item row="2" column="2">
    <widget class="QLabel" name="label_3">
     <property name="styleSheet">
      <string notr="true">background: white; margin-top: 6px</string>
     </property>
     <property name="frameShape">
      <enum>QFrame::StyledPanel</enum>
     </property>
    </widget>
   </item>
   <item row="0" column="0" colspan="3">
    <widget class="QLabel" name="label">
     <property name="styleSheet">
      <string notr="true">background: white; margin-bottom: 6px</string>
     </property>
     <property name="frameShape">
      <enum>QFrame::StyledPanel</enum>
     </property>
    </widget>
   </item>
  </layout>
 </widget>
 <resources/>
 <connections/>
</ui>