What is the link to the film title?

Soundtracks
Impromptu Op. 29 in A Flat
Music by Frédéric Chopin

I found a clue to the tile, other than the obvious musical reference, in a interview with Sarah Kernochan, who wrote the screenplay. She said she had wanted to make a movie that explored "how do complicated people find a simple way of loving?" Impromptu means the opposite of constructed or planned in advance, and kind of fits in perfectly with that theme, don't you think?

Also it's a bit ironic as well because Sand is really a master strategist and takes great care to plan in advance how to woo and win Chopin.

In addition to the existing replies - Chopin himself makes reference to his disappointment that his playing a certain piece, which was supposed to be or seem Impromptu, or extemporaneous, just feels to him like every move he makes can be guessed by his audience, and thus seems planned out, and not Impromptu.