Expression editor gets into unresponsive state

[imtayadeway]

There appears to be some regression in the expression editor.

To illustrate, here I'm about to add a filter to the Compute/Infrastructure/Vms page:

Here I've created a valid expression:

However, on clicking the tick, nothing happens.

I'm using Firefox in the example above. I've tried using other browsers (Chromium, GNU Web) with a little more luck, but after successfully adding an expression to say the Reports filter tab, I'm unable to edit it (I can create the expression, but clicking on the tick fails to update).

/cc @AparnaKarve