Chagall likes a dreamy atmosphere in his paintings (his characters are often floating in the air).
I think the greenish hues on the faces contribute to the irreality here: don’t the persons seem to be already part of the nature outside, after having hastily folded the curtain?
See also that conspicuous curtain, wonderfully supple in this very geometrically constructed scene: an invitation to freedom.
Thank you for the link to this russian site, vanoola.
Marc Chagall,’Window at the Dacha’, 1915, oil and gouache on cardboard.