Arrange your browser window so you can see the Jmol Console also.
This page has a pickCallback() function that echos the pick count
in the upper left. Enter set debugscript to show the pickCallback()
commands in the Jmol Console.
Single clicking atoms will simply increment the pick count once
per click. Note that there is only one pickCallback per double click
as well.
Double-clicking on two different atoms fixes a black distance monitor.
This works in all 3 versions of Jmol unless the pickCallback()
function executes a 'set picking ident' command, in which case it works in Jmol 12 but fails in Jmol 13. Check the checkbox to test this behavior.
Double click on an atom, then move the cursor outside of Jmol. The red monitor line disappears in Jmol 11 but not in Jmol 12 or 13.