Use the mid-point of a stencil mask to determine its visibility on the page.

This works around a rounding error in Chrome where:

  margin-left: 2.6em;
  text-indent: -2.6em;

Is fractionally less than zero, so the rects are shown on the previous page.
Don't set font-size of SVG elements.
Typo in Gala prevented contact events from working in IE<9.