title: Layer-backed NSTextView doing lots of extra drawing with drawsBackground false
body: <p>I'm running into an issue where my NSTextView is doing a tremendous amount of work drawing while idle. The drawing is being triggered by the flashing cursor. This is normal behavior, but the amount of drawing work being done is excessive. If I override <code>shouldDrawInsertionPoint</code> to return false, the work completely stops.</p>
  
  <p>As far as I can tell, the frequency of drawing has not increased, just the amount of work needed to do it. The work done by the CPU <strong>does not</strong> scale with the size of the NSTextView frame, but with the amount of text in the NSTextStorage. And I can see from an Instruments trace that it's mostly NSLayoutManager doing work to layout and draw text. This really surprised me - it certainly appears to be drawing text that is not visible.</p>
  
  <p>I've narrowed down the conditions to an NSTextView with <code>drawsBackground</code> set to false, and <code>wantsLayer</code> set to true. If I change either of these properties, the CPU required to draw the flashing cursor goes way, way down. It is possible for me to work around this by just drawing the background, but I would like to understand this better.</p>
  
  <p>Does anyone have any idea what's up?</p>