Using TextField Cuases GUILayout.Window To Flicker

I'm displaying a small window to let the user enter a name for his new character. The window looks great until the user interacts with the TextField. With every input event (mouse and keyboard), the window momentarily shrinks and then re-expands to its original size.

Here's how it normally looks:

And this is how it looks when you click or type in the TextField:

It immediately snaps back to the original size, hince the "flicker". Here's my code:

 windowRect.width = 350;
 windowRect.height = 200;
 windowRect.x = h - (windowRect.width / 2);
 windowRect.y = v - (windowRect.height / 2);
 windowRect = GUILayout.Window (0, windowRect, DrawPromptNameDialog, "");
 
 ...
 
 void DrawPromptNameDialog (int windowID)
   {
     GUILayout.Space (50);
     GUILayout.Label ("Enter character name:");
     characterName = GUILayout.TextField (characterName);
     GUILayout.BeginHorizontal ();
     {
       if (GUILayout.Button ("Continue", "SmallButton", GUILayout.ExpandWidth (true)))
       {
         // TODO: Validate name
         state = State.PromptSkipTutorial;
       }
       if (GUILayout.Button ("Cancel", "SmallButton", GUILayout.ExpandWidth (true))) {
         state = State.Menu;
       }
     }
     GUILayout.EndHorizontal ();
   }