Cursor disappears when posting

[trashbat]

I go to reply to a thread, and occasionally, the flashing cursor/caret fails to appear in the text box, although I can still enter content that appears wherever I clicked.

I had thought this was an issue with my Android phone (using Chrome), resolved by restarting the browser, but it only ever happens on PH and I've now just seen the same thing on my PC (Firefox).

Dodgy Javascript? Anyone else?