Only hide the browser selection once edit-mode is enabled

The recent shadow cursor implementation hides the native browser selection
highlights. However, the shadow cursor itself only starts being drawn
after edit-mode is enabled, meaning for a readonly document the selection
is invisible (#119)

This is a simple workaround that allows the native selection to be drawn
in circumstances where the shadow cursor is not active. Long term, the
shadow cursor should be also used on non-editable documents as detailed
in solution ideas for #119.