anchor.diff:  * height of listing block not calculated correctly when string of contiguous characters exceeds length of line
    ** put listing block with very long line inside of sidebar; see that sidebar height is incorrect (too large)
    ** is this because source highlighting is not done in scratch document?
  +* add line swell when drawing dashed line on listing block
   * rework resolve_image_path API so it's more logical; override based on type of first argument; document as option
   * rename text-alignment attribute to text-align? (change is within alpha.14, so still a chance to change)
   * report cursor / bounding box bug in column_box
  
positive.code: 
  * QUESTION should we show keys with hyphens instead of underscores in theming guide?
  
  * toclevels=0 should show parts only (to be consistent with HTML); currently shows nothing (#783)
  * define sum on core_ext/array to optimize for Ruby 2.4
  * allow border bottom of table header row to be set by theme
  
negative.code: * conum should never wrap (push it into the text if necessary)
  * decouple theme settings for section titles and discrete headings
  * decouple listing/literal/source theme settings; currently all under code
   ** separate theme control for listing vs literal block (and maybe source too)
  * replace explicit char ranges with classes in regexp (e.g., [[:word:]] or \w)
  * devise a way to specify a value as a string literal (variable replacement only) in theme
  * apply calculated theme values after loading?
  