title: Get "usable" window width in vim script
body: <p><img src="https://i.stack.imgur.com/HRwGU.png" alt="get the width of text field in vim"></p>
  
  <p>How do I get the width of 3 (marked with green color in the image) in vim script?</p>
  
  <p>If there is no <code>signs</code> column, and there are no other "special columns", I can get it with </p>
  
  <p><code>winwidth(0) - (max([len(line('$')), &amp;numberwidth-1]) + 1)</code></p>