Sticky tooltips

[tuner]

Use case: The user would like to copy something from the tooltip. Copying is currently impossible because text cannot be selected from the tooltip.

An alternative behaviour should be provided: the user clicks a mark instance and the tooltip becomes sticky, i.e., it doesn't move, update, or disappear before the user either: clicks somewhere outside the tooltip or clicks an X icon that appeared to the upper right corner of the tooltip. However, this behavior may be undesirable when a custom click handler has been registered and thus, an option for disabling the behavior should be provided. The default behavior could depend on whether a custom click handler has been registered or not.

[tuner]

The click-to-stick behavior is used in the latest Observable Plot. Looks nice: https://twitter.com/mbostock/status/1661814647354449920