The structure "just as~"

Joplin's opera, Treemonisha, is the story of black orphan girl educated by whites. The opera combines elements of ragtime with black work songs and rousing gospel music. When Joplin published it at his own expense in 1911, it was called "an entirely new form of operatic art."
But the timing was wrong. Joplin had invested his hopes in a work based solely on black music just as ragtime was declining in popularity. The one disastrous performance in New York City in 1915 humiliated him, and two years later he was dead.
[Source: Reading for Results Ninth Edition by Laraine Flemming]
I thought the structure "just as~" is always used for comparison like "like~," but in my example, "just as~" seems like to mean "just when~."
So, I'd like to know here what role "just as~" plays.
Thank you in advance for your help.