"Post"-Modernism

[kishnevi]

I have found a solution to your terminology problem in Charles Rosen's "The Classical Style".

In discussing the period of transition between High Baroque and Classical, he uses the term mannerist on the grounds that every composer of that era wrote in their own individual manner, rather than in a general style such as preceded and followed the period. As examples he gives Gluck, Scarlatti, the three sons of Bach, and Haydn's symphonies from the 1760s.

The same general description applies to the music of the Modern/Postmodern period,  so one could speak of 20th  Century Mannerism.