"Add Media" hangs — found one possible cause

Using my custom theme, the ‘Add Media’ button on posts and pages suddenly stopped working — it just hung / froze, with the little spinner going round and round forever.

A bit of searching on the web found lots of other people with the same issue, with advice to turn off plugins and themes in order to narrow down the cause.

It turned out that I had foolishly added <!-- functions.php starts --> to the top of the theme’s functions.php file. Removing that file cured it.

So I’m documenting it here in case anyone else searches for for the cause of similar symptoms.