Ensure we only init CursorHider once
Merge branch 'mrmr1993-ensure-only-init-cursorhider-once' into post-1.46
Revert 932652086c9ad601c92ef4aae250f9e0b6ad51a6.
Disable CursorHider pending fix for #1345.
Add comment re. Math.sign().
Ensure cursor is hidden on scroll
Disable cursor hiding for Chrome versions <= 39.0.2171.71

There is a bug for earlier versions of Chrome which leaves
`pointer-events: none` stuck enabled for Google+ after a certain amount
of scrolling.