title: ExtJS debugging "[E] Layout run failed" (in a custom component)
body: <p>I have developed a <a href="https://github.com/rixo/GridPicker">custom kind of combo box</a> that uses a grid instead of the standard combo picker (mainly to benefit from buffered rendering with huge data sets). I am now trying to make it compatible with Ext 4.2.1 but I ran into this error:</p>
  
  <pre><code>[E] Layout run failed
  </code></pre>
  
  <p>Please, see the <a href="http://planysphere.fr/ext/GridPicker/examples/index-4.2.1">demo pages</a> for test cases. The error is raised once for each combo, but only the first time it is expanded.</p>
  
  <p>This error didn't happen with 4.2.0 (see <a href="http://planysphere.fr/ext/GridPicker/examples/">demo page with 4.2.0</a>). The breaking changes I had identified in 4.2.1 at the time were about the query filter, not rendering or layout... However, I have already been facing this error with 4.2.0 in a situation where the grid picker was sitting in a window, but it was in a code base with lots of overrides and that used the sandboxed version of Ext4... So I just hopped it was not coming from my component and silenced it (<a href="http://planysphere.fr/ext/GridPicker/examples/benchmark/">another demo page</a> proves that grid picker + window is not enough to trigger the error).</p>
  
  <p>The error doesn't seem to have any side effects, but it makes me feel bad.</p>
  
  <p>Does anyone know what is causing that or, even better, what must be done to prevent it?</p>
  
  <p>Or does someone understand Ext's layout engine well enough to give me some pieces of advice on how to track down this kind of error? Or at least give me reassurance that the error will remain harmless in any situation?</p>