Especially when online leftists throw around "idealism" as a meaningless label to dismiss ideas, and claim to be "materialists" themselves. Same vibes as "facts don't care about your feeling." They actually don't even use the correct definition of the word, using it to mean either idealists as in naive and unrealistic, or as in driven by ideals(yeah Im driven by material only, no ideas).
Engels was the first one who proposed the two camps of idealism and materialism. Later the vulgar "dialectical materialism" of Stalin turned this into a rigid division and unfortunately it caught on. In the end, intelligent idealism is still better than dumb materialism.
EDIT: This post actually shows how hopeless some self proclaimed leftists are, completely ignorant about PHILOSOPHY of all things. Is it really too much to ask to read a single page of Marx, and not parrot some other reactionary philosopher? Absolutely pathetic.
Idealism is when you disagree with me, and materialism is when you won't so much as take out the garbage because “the material conditions aren't right, yet”.
Read books, develop ideas, act on those ideas, repeat. Your ideas don't need a perfect and unassailable scientific foundation to be useful, and nobody shits out a perfectly coherent theory of power on the first try.