Bug 1091299 - It's possible to open the SMS activity menu multiple times

unfortunately, adding a class on body seems to trigger a deep style recalculation which prevents the menu from having a transition, at least on my buri with my data.
I even tried removing the CSS change, and we still have the style recalculation, which sounds weird to me and we could probably file a bug about this.

I tried moving the line after `add('visible')`  but it didn't change anything.

What I tried instead, which worked:
`document.body.style.pointerEvents = 'none'` not here (because this seems to trigger a reflow) but just after `add('visible')`, and `document.body.style.pointerEvents = 'initial'` line 152. It seems to both fix the issue and keep the transition for me.

maybe @Cwiiis would know why adding a class on document.body that isn't in any CSS file should trigger a restyle?