I've switched to using Xamarin.Mac from consuming the open source bits directly.
I have the latest (1.4.22) Xamarin.Mac release. According to the release notes, it fixes this regression. Rolf's comment in that thread indicates it was fixed on or around 19th June.
(As an aside, presumably 19th June was before the release of 1.4.22 - it would be helpful if the release notes included the actual release date as well)
However, from my own testing, this issue is still present in 1.4.22, despite being fixed in monomac at the start of May.
How can it be that 1.4.22 contains the later fix, but not the earlier one?