title: Detect unpost of ttk::combobox
body: <p>I have a Tcl/Tk program. In it I have a combobox, like so:</p>
  
  <pre><code>set cb [ttk::combobox .cb -state readonly -textvariable selection -postcommand [list choices .cb]]
  </code></pre>
  
  <p>The proc <code>choices</code> runs when the combobox posts, which is what I want. My question is, how do I detect when the combobox unposts?</p>
  
  <p>I've tried both binding on <code>&lt;&lt;ComboboxSelected&gt;&gt;</code> and setting a variable trace on <code>selection</code>. The problem with each is that they only fire when the user actually changes the selection. I need some way to always detect when the combobox unposts.</p>
  
  <p>Thanks!</p>
  
  <p><strong>edit</strong></p>
  
  <p><em>What I'm trying to accomplish:</em> When the combobox posts it presents the user with a list of options. I don't expect the user to know what the options mean, therefor I am highlighting the options visually in a different area of my program. I have this highlighting triggering and working well with <code>-postcommand</code>. The issue is to know when to turn the highlighting back off.</p>
  
  <p><code>&lt;&lt;ComboboxSelected&gt;&gt;</code> doesn't fire if the user doesn't change the selected value.</p>
  
  <p><code>&lt;Leave&gt;</code> and <code>&lt;FocusOut&gt;</code> fire too soon (e.g. as soon as the box posts).</p>