Fix an issue where clicking in transparent areas of the bucket bar could close
the sidebar when side-by-side mode is disabled. Such clicks are allowed to
bubble up to `document.body` because the user may be trying to click on a
scrollbar thumb or other element in this area. However we don't want these
clicks to close the sidebar like clicking elsewhere on the document does, as
this can be annoying if the user was trying to click on eg. a sidebar bucket but
missed.

In fixing this, the separate mouse and touch event listeners have been replaced
with a pointer event listener, as this is now supported in all of our target
browsers.