Complex sentence: Pursuing display and art-gallery work, he was a window designer for Saks Fifth Avenue 1928--30 and, while in Europe, had invented the 1924 L + T (Leger und Trager) radical hanging system for galleries and museums.
Split into simpler sentences: Pursuing display and art-gallery work, he was a window designer for Saks Fifth Avenue from 1928 to 1930. Earlier, while he was in Europe, Kiesler had invented the 1924 L + T (Leger und Trager) radical hanging system for galleries and museums.