Unfortunately the babel minify-mangle plugin seems to be abandoned and
in certain circumstances can be very buggy. The only safe options are to
disable it, or to switch to a different minification library.
Not minifying our javascript is not ideal, so this commit updates the
javascript tasks to use a rollup, combined with babel, and terser.
Babel still converts code from ES/UMD/AMD to AMD modules with the
relevant browser support, whilst terser minifies the code.
The rollup bundler handles tracking and creation of sourcemaps, and
supports better parallelisation of the tasks.
Since the upgrade to Node LTS/Gallium requires an upgrade to @babel/core
and eslint, which change the built files anyway, this seems like the
ideal time to make this change.
This was created by the Open University a couple of years ago, and is
very helpful for making dragging work reliably on a range of browsers
and with both mouse and touch-screens.
Sadly, HTML5 drag-drop is not yet ready for prime-time.
