In order to allow a user to select content within an element with the `.ol-selectable` class the `user-select` CSS property should be `text`.
The current value of `auto` results in a computed value of `none` due to the rule `.ol-viewport, .ol-unselectable` specifying `user-select: none`. According to the [the specification](https://www.w3.org/TR/css-ui-4/#valdef-user-select-auto):
> if the computed value of `user-select` on the parent of this element is `none`, the computed value is `none`
Tested manually on Chromium, Firefox and IE11.
caa4a0566e