I had considered that, Alan, but I dismissed that thought as I would expect to see at least a little bit of the Lady's left arm in front of the Window ( Maybe that is her left arm, but it just doesn't look right, to me ) and at a similar angle which would be casting the speculated shadow.
Apologies, Mr. MacRae, I expressed myself clumsily!
I believe this is her actual left arm, in front of the glass. It's in the shade due to her own body